Optical pickup apparatus, objective optical element and optical information recording and/or reproducing apparatus

ABSTRACT

An optical pickup apparatus comprises: a first light source; a second light source; a third light source; and an objective optical element. The objective optical element comprises at least two areas comprising first and second optical path difference providing structures. A third light flux from the third light source passing the first optical path difference providing structure forms first and second best focuses. The third light flux passing the objective optical element forms a spot comprising a central spot portion; an intermediate spot portion; and a peripheral spot portion. The central spot portion is used for recording and/or reproducing information for a third optical disk, and the intermediate and peripheral spot portions are not used for recording and/or reproducing information for the third optical disk. The peripheral spot portion is formed on the third optical disk by the third light flux through the second optical path difference providing structure.

This application is based on Japanese Patent Application Nos.2006-193769 filed on Jul. 14, 2006, and 2006-285298 filed on Oct. 19,2006 in Japanese Patent Office, the entire content of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an optical pickup apparatus, objectiveoptical element and optical information recording reproducing apparatusby which information can be recorded and/or reproduced informationcompatibly for different kinds of optical disks.

In recent years, tendency of a shorter wavelength of laser beam as alight source which has been used to record and/or reproduce informationfor optical disks, has become a main stream. For example, a laser lightsource having 400-420 nm wavelength, such as a blue-violet semiconductorlaser; and a blue-SHG laser which converts wavelength of an infraredsemiconductor laser utilizing a second harmonic wave, have been madepractical. Information of 15-20 GB can be recorded on the optical diskhaving a diameter of 12 cm by using these blue-violet optical sourcesand an objective lens having NA (Numerical aperture) which is the sameas a DVD (Digital Versatile Disc). When NA is increased to 0.85,information of 23-25 GB can be recorded onto the optical disk having adiameter of 12 cm. In this specification, the optical disk and anoptical-magnetic disk using a blue-violet laser light source are called“a high density optical disk”.

Hereupon, the high density optical disk using the objective opticalelement, has larger coma caused due to the skew of the optical disk.Therefore some high density optical disks using the objective opticalelement with NA of 0.85, are designed so that the protective layer hasthinner thickness (which is 0.1 mm, while that of DVD is 0.6 mm) thanthat of DVD to reduce the comma due to the skew. On the other hand, itis sometimes considered that a product, such as an optical diskplayer/recorder, which is capable of only recording/reproducinginformation for the above high-density optical disk is worthless. Takingaccount of a fact that, at present, DVDs and CDs (Compact Disc), ontowhich various kinds of information have been recorded, are on themarket, the value of the product as a high-density optical diskplayer/recorder is increased by, for example, enabling to appropriatelyrecord/reproduce information additionally for DVDs and CDs, which a userpossesses. From these backgrounds, the optical pickup apparatusinstalled in the high-density optical disk player/recorder is requiredto be capable of appropriately recording/reproducing information notonly for a high-density optical disk but also a DVD and a CD.

It can be considered, as a method by which the information can beadequately recorded/reproduced while the compatibility is maintainedalso to anyone of the high density optical disk and DVD and further toCD, a method to selectively switch the optical system for the highdensity optical disk and the optical system for DVD and CD correspondingto the recording density of the optical disk to which the information isrecorded/reproduced. However, it requires a plurality of opticalsystems, which is disadvantageous for the size-reduction and whichincreases the cost.

Accordingly, in order to simplify the structure of the optical pickupapparatus and to intend the reduction of cost, it is preferable to makean optical system for the high density optical disk an optical systemfor DVD and CD into a common optical system, and to reduce the number ofoptical parts contributing the optical pickup apparatus as much aspossible, even when the optical pickup apparatus has compatibility.Then, providing the common objective optical element which is arrangedwith facing an optical disk, is most advantageous for the simplificationof the construction or cost reduction of the optical pickup apparatus.Here, in order to obtain the common objective optical element for pluralkinds of optical disks which use different wavelengths forrecording/reproducing information, it is required that the objectiveoptical system is provided with an optical path difference providingstructure having a wavelength dependency for the spherical aberration,which is formed thereon.

European patent application EP-A 1304689 discloses an objective opticalsystem which has the diffractive structure as an optical path differenceproviding structure and can be commonly used for the high densityoptical disk and the conventional DVD and CD, and also discloses anoptical pickup apparatus in which this objective optical system ismounted.

However, the objective optical element for use in the optical pickupapparatus which compatibly conducts recording and/or reproducinginformation for three different optical disks, which is written in EP-A1304689, has a probability that the light amount used for recordingand/or reproducing information is insufficient, and that proper flare ishardly generated when information is recorded and/or reproduced for CDor DVD, depending on the design specification of the optical pickupapparatus, which are problems.

SUMMARY OF THE INVENTION

The present invention has been attained in view of the aforesaidproblems, and at least one of the following objects can be achieved bythe invention. First, one of the objects is to provide an optical pickupapparatus, an objective optical element and an optical informationrecording and/or reproducing apparatus which enable to generate properflare for recording and/or reproducing information for CD and DVD evenif a single lens is used as an objective optical element, and which canproperly record and/or reproduce information for three different typesin terms of recording density such as a high density optical disk, DVDand CD. Another of the objects is to provide an optical pickupapparatus, an objective optical element and an optical informationrecording and/or reproducing apparatus wherein simplification of thestructure and cost reduction can be realized. In addition, another ofthe objects is to provide an optical pickup apparatus, an objectiveoptical element and an optical information recording and/or reproducingapparatus wherein light utilization efficiency can be enhanced andsufficient light amount can be assured for all of three different typesof optical disks.

According to various embodiments, the present teachings can provide anoptical pickup apparatus for recording and/or reproducing informationfor an optical disk. The optical pickup apparatus can comprise: a firstlight source for emitting a first light flux having a first wavelengthλ1; a second light source for emitting a second light flux having asecond wavelength λ2 (λ2>λ1); a third light source for emitting a thirdlight flux having a third wavelength λ3 (λ3>λ2); and an objectiveoptical element. The objective optical element is provided forconverging the first light flux onto an information recording surface ofa first optical disk having a protective substrate with a thickness t1,for converging the second light flux onto an information recordingsurface of a second optical disk having a protective substrate with athickness t2 (t1≦t2), and for converging the third light flux onto aninformation recording surface of a third optical disk having aprotective substrate with a thickness t3 (t2<t3). The optical pickupapparatus can record and/or reproduce information by converging thefirst light flux onto the information recording surface of the firstoptical disk, by converging the second light flux onto the informationrecording surface of the second optical disk, and by converging thethird light flux onto the information recording surface of the thirdoptical disk. The objective optical element can comprise an opticalsurface comprising a central area and a peripheral area surrounding thecentral area. The central area can comprise a first optical pathdifference providing structure, and the peripheral area can comprise asecond optical path difference providing structure. The objectiveoptical element can converge the first light flux which passes throughthe central area of the objective optical element onto the informationrecording surface of the first optical disk so that the optical pickupapparatus can record and/or reproduce information on the informationrecording surface of the first optical disk. The objective opticalelement also can converge the second light flux which passes through thecentral area of the objective optical element onto the informationrecording surface of the second optical disk so that the optical pickupapparatus can record and/or reproduce information on the informationrecording surface of the second optical disk. The objective opticalelement further can converge the third light flux which passes throughthe central area of the objective optical element onto the informationrecording surface of the third optical disk so that the optical pickupapparatus can record and/or reproduce information on the informationrecording surface of the third optical disk. The objective opticalelement can converge the first light flux which passes through theperipheral area of the objective optical element onto the informationrecording surface of the first optical disk so that the optical pickupapparatus can record and/or reproduce information on the informationrecording surface of the first optical disk. The objective opticalelement also can converge the second light flux which passes through theperipheral area of the objective optical element onto the informationrecording surface of the second optical disk so that the optical pickupapparatus can record and/or reproduce information on the informationrecording surface of the second optical disk. In the optical pickupapparatus, the third light flux which passes through the first opticalpath difference providing structure can form a first best focus in whichthe third light flux forms a spot having a smallest diameter, and thethird light flux which passes through the first optical path differenceproviding structure can form a second best focus in which the thirdlight flux forms a spot having a second smallest diameter. The firstbest focus and the second best focus can satisfy the followingexpression:

0<L/f<0.05,

where f (mm) is a focal length of the objective optical element for thethird light flux which passes through the first optical path differencestructure and forms the first best focus, and L (mm) is a distancebetween the first best focus and the second best focus. In the opticalpickup apparatus, the third light flux which passes through theobjective optical element can form a spot on the information recordingsurface of the third optical disk, and the spot can comprise, in orderfrom a center to an outside of the spot when viewing the spot from adirection of an optical axis of the objective optical element: a centralspot portion having a highest light density; an intermediate spotportion having a lower light density than the central spot portion; anda peripheral spot portion having a higher light density than theintermediate spot portion and having a lower light density than thecentral spot portion. The central spot portion can be used for recordingand/or reproducing information for the third optical disk, theintermediate spot portion and the peripheral spot portion can not beused for recording and/or reproducing information for the third opticaldisk. The peripheral spot portion can be formed on the informationrecording surface of the third optical disk by the third light fluxwhich passes through the second optical path difference providingstructure of the objective optical element.

These and other objects, features and advantages according to thepresent invention will become more apparent upon reading of thefollowing detailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements numbered alike in severalFigures, in which:

FIG. 1 is a diagram in which an example of objective optical element OLrelating to the present invention is viewed in the optical axisdirection;

FIG. 2 is a cross-sectional view showing schematically some examples ofan optical path difference providing structure provided on objectiveoptical element OL;

FIG. 3 is a diagram showing a form of a spot relating to the presentinvention;

FIG. 4 is a diagram showing schematically the structure of the opticalpickup apparatus relating to the invention;

FIG. 5 is a cross-sectional view showing schematically an example ofobjective optical element OL relating to the present invention;

FIGS. 6( a)-6(c) are longitudinal spherical aberration diagrams relatingrespectively to BD, DVD and CD in Example 1 of the present invention;

FIGS. 7( a)-7(c) are longitudinal spherical aberration diagrams relatingrespectively to BD, DVD and CD in Example 2 of the present invention;and

FIGS. 8( a)-8(c) are longitudinal spherical aberration diagrams relatingrespectively to BD, DVD and CD in Example 3 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An optical pickup apparatus according to the present invention comprisesat least three light sources: a first light source, a second lightsource, and a third light source. The optical pickup apparatus furthercomprises a light converging optical system for converging the firstlight flux on the information recording surface of the first opticaldisk, converging the second light flux on the information recordingsurface of the second optical disk, and converging the third light fluxon the information recording surface of the third optical disk. Theoptical pickup apparatus according to the present invention furthercomprises a light receiving element for receiving each of reflectedlight fluxes from the information recording surface of the first opticaldisk, the second optical disk, and the third optical disk.

The first optical disk comprises a protective substrate with a thicknessof t1 and an information recording surface. The second optical diskcomprises a protective substrate with a thickness of t2 (t1≦t2) and aninformation recording surface. The third optical disk comprises aprotective substrate of a thickness of t3 (t2<t3) and an informationrecording surface. It is preferable that the first optical disk is ahigh density optical disk, the second optical disk is DVD, and the thirdoptical disk is CD, however, optical disks are not limited to those.Further, in the case where t1<t2, as compared to the case where t1=t2,it is more difficult to record and/or reproduce information for threedifferent optical disks by an objective optical element being a singlelens, with providing excellent tracking characteristics at the time ofrecording and/or reproducing information for the third optical disk.However, an embodiment according to the present invention can conductthat. Hereupon, the first optical disk, the second optical disk or thethird optical disk may also be an optical disk of the plurality oflayers having the plurality of the information recording surfaces.

As an example of the high density optical disk in the presentspecification, there is cited an optical disk (for example, BD: Blu-rayDisc) based on the standard that information is recorded and/orreproduced by an objective optical element with NA 0.85, and that aprotective substrate of the optical disk is about 0.1 mm. Further, as anexample of another high density optical disk, there is cited an opticaldisk (for example, HD DVD: it also called HD) based on the standard thatinformation is recorded and/or reproduced by an objective opticalelement with NA in the range of 0.65 to 0.67 and the protectivesubstrate of the optical disk is about 0.6 mm. Further, the high densityoptical disk includes an optical disk having a protective film (in thepresent specification, the protective substrate includes also theprotective film), having a thickness of about several to several ten nmon the information recording surface, or an optical disk whoseprotective substrate thickness is 0 (zero). The high density opticaldisk further includes a photo-magnetic disk for which the blue-violetsemiconductor laser or blue-violet SHG laser is used as the light sourcefor recording/reproducing information. Further, DVD in the presentspecification represents a generic name of optical disks based on thestandard that information is recorded and/or reproduced by an objectiveoptical element with NA in the range of 0.60 to 0.67 and that theprotective substrate of the optical disk is about 0.6 mm, which belongto DVD group such as DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R,DVD-RW, DVD+R and DVD+RW. In the present specification, CD represents ageneric name of optical disks based on the standard that information isrecorded and/or reproduced by an objective optical element with NA inthe range of 0.45 to 0.53 and that the protective substrate of theoptical disk is about 1.2 mm, which belong to CD group such as CD-ROM,CD-Audio, CD-Video, CD-R and CD-RW. Among these optical disks, the highdensity optical disk provides the highest recording density. DVD and CDprovide the second highest recording density, the third highestrecording density, respectively.

Thicknesses t1, t2, and t3 of the protective substrates preferablysatisfy the following conditional expressions (11), (12), and (13).However, it is to be understood that various changes and modificationswill be apparent to those skilled in the art.

0.0750 mm≦t1≦0.125 mm or 0.5 mm≦t1≦0.7 mm  (11)

0.5 mm≦t2≦0.7 mm  (12)

1.0 mm≦t3≦1.3 mm  (13)

In the present specification, each of the first light source, the secondlight source, and the third light source is preferably a laser lightsource. A semiconductor laser, and a silicon laser are preferably usedfor the laser light source. The first wavelength λ1 of the first lightflux emitted from the first light source, the second wavelength λ2(λ2>λ1) of the second light flux emitted from the second light source,the third wavelength λ3 (λ3>λ2) of the third light flux emitted from thethird light source, are preferable to satisfy the following conditionalexpressions (9) and (10).

1.5×λ1<λ2<1.7×λ1  (9)

1.9×λ1<λ3<2.1×λ1  (10)

When BD or HD is employed as the first optical disk, the wavelength λ1of the first light source is preferably 350 nm or more, and 440 nm orless. The wavelength λ1 is more preferably 380 nm or more, and 415 nm orless. When DVD is employed as the second optical disk, the secondwavelength λ2 of the second light source is preferably 570 nm or more,and 680 nm or less. The second wavelength λ2 is more preferably 630 nmor more, and 670 nm or less. When CD is employed for the third opticaldisk, the third wavelength λ3 of the third light source is preferably750 nm or more, and 880 nm or less. The third wavelength λ3 is morepreferably 760 nm or more, and 820 nm or less.

Further, at least two light sources of the first light source, thesecond light source, and the third light source may also be unitized.The unitization means fixing and housing. For example, the first lightsource and the second light source can be into one package. However itis to be understood that various changes and modifications will beapparent to those skilled in the art. The unitization in a broad senseincludes a situation that two light sources are fixed so that aberrationcan not be corrected. Further, in addition to the light source, thelight receiving element which will be described later, may also beprovided as one package.

As the light receiving element, the photodetector such as a photo diodeis preferably used. The light reflected on the information recordingsurface of the optical disk enters into the light receiving element, andsignal outputted from the light receiving element is used for obtainingthe read signal of the information recorded in each optical disk.Further, the light amount of the spot on the light receiving elementcaused with the change in the spot shape and the change in the spotposition, to conduct the focus detection and the tracking detectionfocus detection. The objective optical element is moved based on thesedetections for focusing and tracking of the objective optical element.The light receiving element may be composed of a plurality ofphotodetectors. The light receiving element may also have a mainphotodetector and secondary photodetector. For example, the lightreceiving element is provided with a main photodetector which receivesthe main light used for recording and/or reproducing information, andtwo secondary photodetectors positioned on both sides of the mainphotodetector, so as to receive secondary light for tracking adjustmentby the two secondary photodetectors. Further, the light receivingelement may also comprise a plurality of light receiving elementscorresponding to each light source.

The light converging optical system comprises the objective opticalelement. The light converging optical system may comprise only anobjective optical element. Alternatively, the light converging opticalsystem may further comprise a coupling lens such as a collimator lensother than the objective optical element. The coupling lens means asingle lens or a lens group which is arranged between the objectiveoptical element and the light source and which changes divergent angleof a light flux. The collimator lens is a kind of coupling lens and is alens receiving an incident light flux and emitting it as a parallellight flux. Further, the light converging optical system may alsocomprise an optical element such as the diffractive optical elementwhich divides the light flux emitted from the light source into a mainlight flux used for recording reproducing information and two secondarylight fluxes used for the tracking operation and so on. In the presentspecification, the objective optical element means an optical systemwhich is arranged to face the optical disk in the optical pickupapparatus, which has the function which converges the light flux emittedfrom the light source onto an information recording surface of theoptical disk. Preferably, the objective optical element is an opticalsystem which is arranged to face the optical disk in the optical pickupapparatus, and which has the function which converges the light fluxemitted from the light source on the information recording surface ofthe optical disk, and further which is movable as one body in thedirection of at least the optical axis by an actuator. The objectiveoptical element may be formed of a plurality of lenses and/or opticalelements. Alternatively, the objective optical element may be a singlelens. Preferably, the objective lens is formed of a single lens. Theobjective optical element may also be a glass lens, a plastic lens or ahybrid lens in which an optical path difference providing structure isformed on the glass lens by using photo-curing resin. When the objectiveoptical element has a plurality of lenses, a combination of a glass lensand a plastic lens can be used for the objective optical element. Whenthe objective optical element has a plurality of lenses and/or opticalelements, there may be provided a combination of an optical element inflat plate shape having an optical path difference providing structureand an aspheric surface lens which has a optical path differenceproviding structure or does not have the optical path differenceproviding structure. The objective optical element preferably comprisesa refractive surface which is an aspheric surface. Further, theobjective optical element preferably can have a base surface where theoptical path difference providing structure is provided, which is anaspheric surface.

Further, when the objective optical element is a glass lens, a glassmaterial used for the glass lens preferably has a glass transition pointTg of 400° C. or less. By using the glass material whose glasstransition point Tg is 400° C. or less, the material can be molded at acomparatively low temperature. Therefore, the life of the metallic moldcan be prolonged. As an example of the glass material whose glasstransition point Tg is low, there are K-PG325 and K-PG375 (both aretrade names) made by SUMITA Optical glass, Inc.

Hereupon, a glass lens has generally larger specific gravity than aresin lens. Therefore, the objective optical element made of a glasslens has larger weight and applies a larger burden to the actuator whichdrives the objective optical element. Therefore, when a glass lens isemployed for the objective optical lens, a glass material having smallspecific gravity is preferably used for the objective optical element.Specifically, the specific gravity is preferably 3.0 or less, and ismore preferably 2.8 or less.

Further, when a plastic lens is employed for the objective opticalelement, it is preferable that the resin material of cyclic olefins isused for the objective optical element. In the cyclic olefins, there ismore preferably used the resin material having: refractive index at thetemperature 25° C. for wavelength 405 nm, which is within the range of1.54 to 1.60; and ratio of refractive index change dN/dT (° C.⁻¹) withthe temperature change within the temperature range of −5° C. to 70° C.for the wavelength 405 nm, which is within the range of −20×10⁻⁵ to−5×10⁻⁵ (more preferably, −10×10⁻⁵ to −8×10⁻⁵). Further, when a plasticlens is employed for the objective optical element, it is preferablethat a plastic lens is also employed for the coupling lens.

Alternatively, as the resin material appropriate to the objectiveoptical element of the present invention, there is “athermal resin” alsoother than the cyclic olefins. “Athermal resin” is a resin material inwhich microparticles each having a diameter of 30 nm or less aredispersed into a resin which is a base material. The material of themicroparticles has ratio of change in the refractive index with thetemperature change, and the ratio of change in the refractive index ofthe microparticles has the opposite sign to that of the material of theresin which is the base material. Generally, when microparticles aremixed in the transparent resin material, light is scattered and thetransmission factor is lowered. So, it is difficult to use as theoptical material. However, it becomes clear that the microparticleswhose size is smaller than the wavelength of the transmitting light fluxprevent the scattering effectively.

Hereupon, the refractive index of the resin material is lowered when thetemperature rises, while the refractive index of the inorganicmicroparticles is increased when the temperature rises. Accordingly, itis also well known to prevent the refractive index from changing bycombining above nature of the microparticles and the base material so asto cancel them out each other. When the objective optical elementaccording to the present invention employs the material such that theinorganic particles whose size is 30 nanometer or less, which ispreferably 20 nanometer or less, more preferably 10-15 nanometers, aredispersed in the resin as base material, there can be provided theobjective optical unit having no or very low temperature dependency ofthe refractive index.

For example, microparticles of niobium oxide (Nb₂O₅) are dispersed inacryl resin. The volume ratio of the resin material that represents thebasic material is about 80% and that of niobium oxide is about 20%, andthese are mixed uniformly. Though microparticles have a problem thatthey tend to condense, the necessary state of dispersion can be kept bya technology to disperse particles by giving electric charges to thesurface of each particle.

It is preferable that microparticles are mixed and dispersed into theresin as a base material during injection molding of optical elements bythe in-line manner. In other words, it is preferable that, after themicroparticles are mixed and dispersed in to the base material, themixture is neither cooled nor solidified until the mixture is moldedinto an objective optical unit.

Incidentally, in order to control the ratio of change in the refractiveindex with the temperature, the volume ratio of microparticles to thebase material may increase or decrease, and microparticles in which aplural kinds of nanometer-sized microparticles are blended may also bedispersed into the base material.

Though the volume ratio of the microparticles and the base material ismade to be 80:20, namely to be 4:1, in the example stated above, it ispossible to adjust properly within a range from 90:10 (9:1) to 60:40(3:2). It is preferable that a volume of the microparticles is providedto be exceed the ratio of 9:1, because the temperature-affected changeis effectively reduced. While, it is also preferable that a volume ofthe microparticles is provided to be less than the ratio of 3:2, becausemoldability of the athermal resin becomes easy.

It is preferable that the microparticles are inorganic substances, andmore preferable that the microparticles are oxides. Further, it ispreferable that the state of oxidation is saturated, and the oxides arenot oxidized any more.

It is preferable that the microparticles are inorganic substancesbecause reaction between the inorganic substances and resin as a basematerial representing high molecular organic compound is restrained tobe low, and deterioration caused by actual use such as irradiation oflaser beam can be prevented because the microparticles are oxides. Inparticular, under the severe conditions such as high temperature andirradiation of a laser beam, oxidation of resin tends to be accelerated.However, aforesaid microparticles of inorganic oxide are prevented fromdeterioration caused by oxidation.

Further, it is naturally possible to add antioxidants in resin materialin order to prevent the resin from oxidation caused by other factors.

Materials described in JP-A 2004-144951, JP-A 2004-144953, JP-A2004-144954 are suitable for a preferable material to be base material.

Inorganic microparticles to be dispersed in thermoplastic resin are notlimited in particular, and suitable microparticles can be arbitrarilyselected from inorganic microparticles which reduce the ratio(hereinafter, |dn/dT|) of change in refractive index with thetemperature. To be concrete, oxide microparticles, metal saltmicroparticles and semiconductor microparticles are preferably used, andit is preferable to use by selecting properly those in which absorption,light emission and fluorescence are not generated in the wavelengthrange employed for an optical element, from the aforesaidmicroparticles.

The following metal oxide is used for oxide microparticles used in thestructure according to the present invention: a metal oxide constructedby one or more kinds of metal selected by a group including Li, Na, Mg,Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Sr, Y, Nb, Zr,Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, La, Ta, Hf, W, Ir, Tl, Pb, Bi and rareearth metal. More specifically, for example, oxide such as siliconoxide, titanium oxide, zinc oxide, aluminum oxide, zirconium oxide,hafnium oxide, niobium oxide, tantalum oxide, magnesium oxide, calciumoxide, strontium oxide, barium oxide, indium oxide, tin oxide, leadoxide; complex oxide compounds these oxides such as lithium niobate,potassium niobate and lithium tantalate, the aluminum magnesium oxide(MgAl₂O₄) are cited. Furthermore, rare earth oxides are used for theoxide microparticles in the structure according to the presentinvention. More specifically, for example, scandium oxide, yttriumoxide, lanthanum trioxide, cerium oxide, praseodymium oxide, neodymiumoxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide,dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbiumoxide, lutetium oxide are cited. As metal salt microparticles, thecarbonate, phosphate, sulfate, etc. are cited. More specifically, forexample, calcium carbonate, aluminum phosphate are cited.

Moreover, semiconductor microparticles in the structure according to thepresent invention mean the microparticles constructed by asemiconducting crystal. The semiconducting crystal composition examplesinclude simple substances of the 14th group elements in the periodictable such as carbon, silica, germanium and tin; simple substances ofthe 15th group elements in the periodic table such as phosphor (blackphosphor); simple substances of the 16th group elements in the periodictable such as selenium and tellurium; compounds comprising a pluralnumber of the 14th group elements in the periodic table such as siliconcarbide (SiC); compounds of an element of the 14th group in the periodictable and an element of the 16th group in the periodic table such as tinoxide (IV) (SnO₂), tin sulfide (II, IV) (Sn(II) Sn(IV) S₃), tin sulfide(IV) (SnS₂), tin sulfide (II) (SnS), tin selenide (II) (SnSe), tintelluride (II) (SnTe), lead sulfide (II) (PbS), lead selenide (II)(PbSe) and lead telluride (II) (PbTe); compounds of an element of the13th group in the periodic table and an element of the 15th group in theperiodic table (or III-V group compound semiconductors) such as boronnitride (BN), boron phosphide (BP), boron arsenide (BAs), aluminumnitride (AlN), aluminum phosphide (AlP), aluminum arsenide (AlAs),aluminu antimonide (AlSb), gallium nitride (GaN), gallium phosphide(GaP), gallium arsenide (GaAs), gallium antimonide (GaSb), indiumnitride (InN), indium phophide (InP), indium arsenide (InAs) and indiumantimonide (InSb); compounds of an element of the 13th group in theperiodic table and an element of the 16th group in the periodic tablesuch as aluminum sulfide (Al₂S₃), aluminum selenide (Al₂Se₃), galliumsulfide (Ga₂S₃), gallium selenide (Ga₂Se₃), gallium telluride (Ga₂Te₃),indium oxide (In₂O₃), indium sulfide (In₂S₃), indium selenide (In₂Se₃)and indium telluride (In₂Te₃); compounds of an element of the 13th groupin the periodic table and an element of the 16th group in the periodictable such as thallium chloride (I) (TlCl), thallium bromide (I) (TlBr),thallium iodide (I) (TlI); compounds of an element of the 12th group inthe periodic table and an element of the 16th group in the periodictable (or II-VI group compound semiconductors) such as zinc oxide (ZnO),zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), cadmiumoxide (CdO), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmiumtelluride (CdTe), mercury sulfide (HgS), mercury selenide (HgSe) andmercury telluride (HgTe); compounds of an element of the 15th group inthe periodic table and an element of the 16th group in the periodictable such as arsenic sulfide (III) (As₂S₃), arsenic selenide (III)(As₂Se₃), arsenic telluride (III) (As₂Te₃), antimony sulfide (III)(Sb₂S₃), antimony selenide (III) (Sb₂Se₃), antimony telluride (III)(Sb₂Te₃), bismuth sulfide (III) (Bi₂S₃), bismuth selenide (III) (Bi₂Se₃)and bismuth telluride (III) (Bi₂Te₃); compounds of an element of the11th group in the periodic table and an element of the 16th group in theperiodic table such as copper oxide (I) (Cu₂O) and copper selenide (I)(Cu₂Se); compounds of an element of the 11th group in the periodic tableand an element of the 17th group in the periodic table such as copperchloride (I) (CuCl), copper bromide (I) (CuBr), copper iodide (I) (CuI),silver chloride (AgCl) and silver bromide (AgBr); compounds of anelement of the 10th group in the periodic table and an element of the16th group in the periodic table such as nickel oxide (II) (NiO);compounds of an element of the 9th group in the periodic table and anelement of the 16th group in the periodic table such as cobalt oxide(II) (CoO) and cobalt sulfide (II) (CoS); compounds of an element of the8th group in the periodic table and an element of the 16th group in theperiodic table such as triiron tetraoxide (Fe₃O₄) and iron sulfide (II)(FeS); compounds of an element of the 7th group in the periodic tableand an element of the 16th group in the periodic table such as manganeseoxide (II) (MnO); compounds of an element of the 6th group in theperiodic table and an element of the 16th group in the periodic tablesuch as molybdenum sulfide (IV) (MoS₂) and tungsten oxide(IV) (WO₂);compounds of an element of the 5th group in the periodic table and anelement of the 16th group in the periodic table such as vanadium oxide(II) (VO), vanadium oxide (IV) (VO₂) and tantalum oxide (V) (Ta₂O₅);compounds of an element of the 4th group in the periodic table and anelement of the 16th group in the periodic table such as titanium oxide(such as TiO₂, Ti₂O₅, Ti₂O₃ and Ti₅O₉); compounds of an element of the2th group in the periodic table and an element of the 16th group in theperiodic table such as magnesium sulfide (MgS) and magnesium selenide(MgSe); chalcogen spinels such as cadmium oxide (II) chromium (III)(CdCr₂O₄), cadmium selenide (II) chromium (III) (CdCr₂Se₄), coppersulfide (II) chromium (III) (CuCr₂S₄) and mercury selenide (II) chromium(III) (HgCr₂Se₄); and barium titanate (BaTiO₃). Further, semiconductorclusters structures of which are established such as BN₇₅(BF₂)₁₅F₁₅,described in Adv. Mater., vol. 4, p. 494 (1991) by G. Schmid, et al.;and Cu₁₄₆Se₇₃(triethylphosphine)₂₂ described in Angew. Chem. Int. Ed.Engl., vol. 29, p. 1452 (1990) by D. Fenske are also listed as examples.

In general, dn/dT of thermoplastic resin has a negative value, namely, arefractive index becomes smaller as the temperature rises. Therefore, itis preferable to disperse microparticles having large dn/dT, in order tomake |dn/dT| of thermoplastic resin composition to be efficiently small.It is preferable that the absolute value of dn/dT of the microparticlesis smaller than that of the thermoplastic resin used as a base materialwhen using microparticles having dn/dT with same sign to the sign ofdn/dT of the thermoplastic resin. Furthermore, microparticles havingpositive dn/dT, which is microparticles having different sign of dn/dTfrom that of the thermoplastic resin which is a base material, arepreferably used. By dispersing these kinds of microparticles into thethermoplastic resin, |dn/dT| of thermoplastic resin composition caneffectively become small with less amount of the microparticles. It ispossible to properly select dn/dT of microparticles to be dispersedcorresponding to a value of dn/dT of thermoplastic resin to become abase material. However, it is preferable that dn/dT of microparticles isgreater than −20×10⁻⁶ and it is more preferable that dn/dT ofmicroparticles is greater than −10×10⁻⁶ when microparticles aredispersed into a thermoplastic resin which is preferably employed to ageneral optical element. As microparticles having large dn/dT, galliumnitride, zinc sulfate, zinc oxide, lithium niobate and lithiumtantalite, for example, are preferably used.

On the other hand, when dispersing microparticles in thermoplasticresin, it is preferable that a difference of refractive index betweenthe thermoplastic resin to become a base material and the microparticlesis small. Scattering is hardly caused when light is transmitted, if adifference of refractive index between the thermoplastic resin and themicroparticles to be dispersed is small. In case of dispersingmicroparticles in the thermoplastic resin, microparticles in larger sizeeasily cause scattering when light flux transmits the material. However,in a material in which a difference of refractive index between thethermoplastic resin and the microparticles to be dispersed is small, anoccurrence of light scattering becomes low even when relativelylarge-sized microparticles are used. A difference of refractive indexbetween the thermoplastic resin and the microparticles to be dispersedis preferably within the range of 0-0.3, and more preferably within therange of 0-0.15.

Refractive indexes of thermoplastic resins preferably used as opticalmaterials are in the range about 1.4-1.6 in many cases. As materials tobe dispersed in these thermoplastic resins, silica (silicon oxide),calcium carbonate, aluminum phosphate, aluminum oxide, magnesium oxide,and aluminum.magnesium oxides, for example, are preferably used.

Further, dn/dT of thermoplastic resin composition can be made smalleffectively, by dispersing microparticles whose refractive index isrelatively low. As a reason why |dn/dT| of thermoplastic resincomposition including dispersed microparticles with low refractive indexbecomes small, it is considered that temperature changes of the volumefraction of inorganic microparticles in the resin composition may workto make the |dn/dT| of the resin composition to become smaller when therefractive index of the microparticles is lower, although the detailsare not clarified. As microparticles having a relatively low refractiveindex, silica (silicon oxide), calcium carbonate and aluminum phosphate,for example, are preferably used.

It is difficult to simultaneously achieve all of improving an effect oflowering dn/dT of the thermoplastic resin composition, improving oflight transmittance and a desired refractive index. Therefore,microparticles to be dispersed in the thermoplastic resin can beselected properly by considering a magnitude of dn/dT of a microparticleitself, a difference of dn/dT between microparticles and thethermoplastic resin to become a base material, and the refractive indexof the microparticles, depending on the characteristics which arerequired for the thermoplastic resin composition. Further, it ispreferable, for maintaining light transmittance, to properly selectmicroparticles which hardly cause light scattering with considering itsaffinity with the thermoplastic resin to become a base material, inother words, characteristics of the microparticles in dispersion for thethermoplastic resin.

For example, when using cyclic olefin polymer preferably employed for anoptical element as a base material, silica is preferably used asmicroparticles which make |dn/dT| small while keeping lighttransmittance.

For the microparticles mentioned above, it is possible to use either onetype of inorganic microparticles or plural types of inorganicmicroparticles in combination. By using plural types of microparticleseach having a different characteristic, the required characteristics canfurther be improved efficiently.

Inorganic microparticles relating to the present invention preferablyhas an average particle size being 1 nm or larger and being 30 nm orsmaller and more preferably has an average particle size being 1 nm ormore and being 10 nm or less. When the average particle size is lessthan 1 nm, dispersion of the inorganic microparticles is difficult,resulting in a fear that the required efficiency may not be obtained,therefore, it is preferable that the average particle size is 1 nm ormore. When the average particle size exceeds 30 nm, thermoplasticmaterial composition obtained becomes muddy and transparency is lowered,resulting in a fear that the light transmittance may become less than70%, therefore, it is preferable that the average particle size is 30 nmor less. The average particle size mentioned here means volume averagevalue of a diameter (particle size in conversion to sphere) inconversion from each particle into a sphere having the same volume asthat of the particle.

Further, a form of an inorganic microparticle is not limited inparticular, but a spherical microparticle is used preferably. To beconcrete, a range of 0.5-1.0 for the ratio of the minimum size of theparticle (minimum value of the distance between opposing two tangentseach touching the outer circumference of the microparticle)/the maximumsize (maximum value of the distance between opposing two tangents eachtouching the outer circumference of the microparticle) is preferable,and a range of 0.7-1.0 is more preferable.

A distribution of particle sizes is not limited in particular, but arelatively narrow distribution is used suitably, rather than a broaddistribution, for making the invention to exhibit its effectefficiently.

The objective optical element will be described below. At least oneoptical surface of the objective optical element comprises a centralarea and a peripheral area around the central area. More preferably, atleast one optical surface of the objective optical element furthercomprises a most peripheral area around the peripheral area. Byproviding the most peripheral area, it allows to more appropriatelyrecord and/or reproduce information for the optical disk using the highNA. The central area preferably is an area having the optical axis ofthe objective optical element, however, it may also be the area notincluding the optical axis. It is preferable that the central area,peripheral area, and most peripheral area are provided on the sameoptical surface. As shown in FIG. 1, it is preferable that the centralarea CN, peripheral area MD, most peripheral area OT are provided on thesame optical surface concentrically around the optical axis. Further,the first optical path difference providing structure is provided in thecentral area of the objective optical element. The second optical pathdifference providing structure is provided in the peripheral area. Whenthe most peripheral area is provided, the most peripheral area may be arefractive surface, or the third optical path difference providingstructure may be provided in the most peripheral area. It is preferablethat each of the central area, peripheral area, most peripheral areaadjoins to the neighboring area, however, there may be slight gapsbetween adjoining areas.

The area where the first optical path difference providing structure isprovided is preferably 70% or more of the area of the central area onthe objective optical element. It is more preferably 90% or more of thearea of the central area. The first optical path difference providingstructure is furthermore preferably provided on the entire surface ofthe central area. The area where the second optical path differenceproviding structure is provided is preferably 70% or more of theperipheral area on the objective optical element. It is more preferably90% or more of the area of the peripheral area. The second optical pathdifference providing structure is further more preferably provided onthe entire surface of the peripheral area. The area where the thirdoptical path difference providing structure is provided, is 70% or moreof the area of the most peripheral area on the objective opticalelement. It is more preferably 90% or more of the area of the mostperipheral area. The third optical path difference providing structureis more preferably provided on the entire surface of the most peripheralarea.

Hereupon, the optical path difference providing structure used in thepresent specification, is the general name of the structure by which anoptical path difference is provided to an incident light flux. Theoptical path difference providing structure also includes the phasedifference providing structure by which the phase difference isprovided. Further, the phase difference providing structure includes adiffractive structure. The optical path difference providing structurecomprises a step, preferably, comprises a plurality of steps. This stepprovides an optical path difference and/or phase difference to anincident light flux. The optical path difference added by the opticalpath difference providing structure may also be an integer times of thewavelength of the incident light flux, or may also be non-integer timesof the wavelength of the incident light flux. The step may also bearranged with periodic interval in the direction perpendicular to theoptical axis, or may also be arranged with non-periodic interval in thedirection perpendicular to the optical axis.

It is preferable that the optical path difference providing structurecomprises a plurality of ring-shaped zones arranged concentricallyaround the optical axis. Further, the optical path difference providingstructure can have various sectional shapes (cross sectional shapes inthe plane including the optical axis). One of the most common opticalpath difference providing structure provides the sectional shapeincluding the optical axis, which is in the serrated shape, as shows inFIG. 2( a). Even when the cross sectional shape of the optical pathdifference providing structure arranged on the flat plane looks astepped shape, the same optical path difference providing structurearranged on an aspheric surface can be considered as the serrated shapeshown in FIG. 2( a). Accordingly, in the present specification, it isdefined that the sectional shape in the serrated shape includes thesectional shape in the stepped shape. Each of the first optical pathdifference providing structure and the second optical path differenceproviding structure of the present specification, may have a sectionalshape which is formed by overlapping different optical path differenceproviding structures having serrated shape. For example, FIG. 2( b)shows the structure in which the fine serrated shaped optical pathdifference providing structure and the rough serrated optical pathdifference providing structure overlap with each other.

Further, the first optical path difference providing structure providedin the central area of the objective optical element and the secondoptical path difference providing structure provided in the peripheralarea of the objective optical element may be provided on the differentoptical surface of the objective optical element. However, it ispreferable that the first and second optical path difference providingstructures are provided on the same optical surface. By providing themon the same optical surface, it reduces the decentration error at thetime of the manufacture, which is preferable. Further, it is preferablethat the first optical path difference providing structure and thesecond optical path difference providing structure are provided on thesurface on the light source side of the objective optical element,rather than the surface on the optical disk side of the objectiveoptical element. Further, when the objective optical element comprises amost peripheral area including the third optical path differenceproviding structure, it is preferable that also the third optical pathdifference providing structure is arranged on the same optical surfaceto that of the first and second optical path difference providingstructure.

When the objective optical element has a fourth optical path differenceproviding structure, it is preferable that the fourth optical pathdifference providing structure is provided on an optical surface whichis different from an optical surface on which a first optical pathdifference providing structure and a second optical path differenceproviding structure are provided. It is further preferable that thefourth optical path difference providing structure is provided on thesurface of the objective optical element closer to an optical disk.

The objective optical element converges a first light flux, a secondlight flux and a third light flux which pass through a central areawhere the first optical path difference providing structure of theobjective optical element is provided, so that each of them may form aconverged spot. Preferably, the objective optical element converges afirst light flux which passes through a central area where the firstoptical path difference providing structure of the objective opticalelement is provided, so that recording and/or reproducing of informationmay be conducted on an information recording surface of the firstoptical disk. Further, the objective optical element converges a secondlight flux which passes through a central area where the first opticalpath difference providing structure of the objective optical element isprovided, so that recording and/or reproducing of information may beconducted on an information recording surface of the second opticaldisk. Further, the objective optical element converges a third lightflux which passes through a central area where the first optical pathdifference providing structure of the objective optical element isprovided, so that recording and/or reproducing of information may beconducted on an information recording surface of the third optical disk.When thickness t1 of a protective substrate of a first optical disk isdifferent from thickness t2 of a protective substrate of a secondoptical disk, it is preferable that the first optical path differenceproviding structure corrects spherical aberration caused by a differencebetween thickness t1 of a protective substrate of the first optical diskand thickness t2 of a protective substrate of the second optical diskand/or spherical aberration caused by a difference of wavelength betweenthe first light flux and the second light flux, for the first light fluxand the second light flux which pass through the first optical pathdifference providing structure. In addition, it is preferable that thefirst optical path difference providing structure corrects sphericalaberration caused by a difference between thickness t1 of a protectivesubstrate of the first optical disk and thickness t3 of a protectivesubstrate of the third optical disk and/or spherical aberration causedby a difference of wavelength between the first light flux and the thirdlight flux, for the first light flux and the third light flux which passthrough the first optical path difference providing structure.

Further, the third light flux which passes through the first opticalpath difference providing structure of the objective optical elementforms the first best focus where a spot diameter of the spot formed bythe third light flux is the smallest and the second best focus where aspot diameter of the spot formed by is the second smallest to the firstbest focus. Incidentally, it is assumed that the best focus mentioned inthis case means a point where beam-waist is minimum within a certainrange of defocus. In other words, forming of the first best focus andthe second best focus by the third light flux means that at least twopoints where the beam-waist is minimum within a certain range of defocusexist in the third light flux. Incidentally, it is preferable that, inthe third light flux passing the first optical path providing structure,a diffracted light having the maximum light amount forms the first bestfocus, and a diffracted light having the second largest light amountforms the second best focus. Further, when the diffraction efficiency ofa diffracted light forming the first best focus is greater than that ofthe diffracted light forming the second best focus, and when adifference between them is 15% or more (preferably, 30% or more), aneffect of the invention that light utilization efficiency can beenhanced in the third light flux becomes more conspicuous.

It is preferable that a spot formed by the third light flux at the firstbest focus is used for recording and/or reproducing for the thirdoptical disk and a spot formed by the third light flux at the secondbest focus is not used for recording and/or reproducing for the thirdoptical disk. However, an embodiment wherein a spot formed by the thirdlight flux at the first best focus is not used for recording and/orreproducing for the third optical disk and a spot formed by the thirdlight flux at the second best focus is used for recording and/orreproducing for the third optical disk, is not denied. Meanwhile, whenthe first optical path difference providing structure is provided on thesurface of the objective optical element facing a light source, thereare considered two cases including one occasion where the second bestfocus is closer to the objective optical element than the first bestfocus is and another occasion where the second best focus is fartherfrom the objective optical element than the first best focus is. Forexample, when a zero-th order diffracted light of the third light fluxforms the second best focus, a position of the second best focus iscloser to the objective optical element than that of the first bestfocus formed by the first order diffracted light of the third lightflux. On the other hand, when the second best focus is formed by thesecond order diffracted light of the third light flux, a position of thesecond best focus is farther from the objective optical element thanthat of the first best focus formed by the first order diffracted lightof the third light flux.

Further, the first best focus and the second best focus satisfy thefollowing expression (1):

0<L/f<0.05  (1)

Where, f [mm] represents a focal length of the third light flux thatpasses through the first optical path difference providing structure andforms the first best focus, while, L [mm] represents a distance betweenthe first best focus and the second best focus.

Incidentally, it is more preferable to satisfy the following expression(1′).

0.01≦L/f≦0.043  (1′)

More preferable is to satisfy the following expression (1″).

0.016≦L/f≦0.042  (1″)

Further, L is preferably 0.03 mm or more, and 0.11 mm or less, and f ispreferably 1.8 mm or more, and 3.0 mm or less.

It is preferable to satisfy the aforesaid expression (1), (1′) or (1″),both for improving the light utilization efficiency for recording and/orreproducing for the third optical disk and for maintaining sufficientlight utilization efficiency for recording and/or reproducinginformation for the first optical disk and the second optical disk.

The objective optical element converges the first light flux and thesecond light flux each of which passes through a peripheral area wherethe second optical path difference providing structure of the objectiveoptical element is arranged, so as to form a converged spot. Preferably,the objective optical element converges the first light flux that passesthrough a peripheral area where the second optical path differenceproviding structure of the objective optical element is arranged so asto record and/or reproduce information on the information recordingsurface of the first optical disk. Further, the objective opticalelement converges the second light flux that passes through a peripheralarea where the second optical path difference providing structure of theobjective optical element is arranged so as to record and/or reproduceinformation on an information recording surface of the second opticaldisk. Further, when thickness t1 of a protective substrate of a firstoptical disk is different from thickness t2 of a protective substrate ofa second optical disk, it is preferable that the second optical pathdifference providing structure corrects spherical aberration caused by adifference between thickness t1 of a protective substrate of the firstoptical disk and thickness t2 of a protective substrate of the secondoptical disk and/or spherical aberration caused by a difference ofwavelength between the first light flux and the second light flux, forthe first light flux and the second light flux which pass through thesecond optical path difference providing structure.

Further, as a preferable embodiment, there is given an embodimentwherein the third light flux which passes the peripheral area is notused for recording and/or reproducing information for the third opticaldisk. It is preferable to make the third light flux passing through aperipheral area not to contribute to forming of a converged spot on aninformation recording surface of the third optical disk. In other words,it is preferable that the third light flux passing through a peripheralarea where the second optical path difference providing structure of theobjective optical element is provided forms flare on an informationrecording surface of the third optical disk. As shown in FIG. 3, a spotformed on an information recording surface of the third optical disk bythe third light flux passing the objective optical element comprisescentral spot portion SCN having high light density, intermediate spotportion SMD having light density that is lower than that of the centralspot portion and peripheral spot portion SOT having light density thatis higher than that of the intermediate spot portion and is lower thanthat of the central spot portion, in this order from the optical axisside (or central spot portion) to the outer side. The central spotportion is used for recording and/or reproducing of information for anoptical disk, while, the intermediate spot portion and the peripheralspot portion are not used for recording and/or reproducing ofinformation for an optical disk. In the foregoing, this peripheral spotportion is called a flare. In other words, the third light flux whichpasses through the second optical path difference providing structureprovided on a peripheral area of the objective optical element forms aperipheral spot portion on an information recording surface of the thirdoptical disk. Incidentally, it is preferable that a converged spot or aspot of the third light flux mentioned in this case is a spot at thefirst best focus. Further, it is preferable that a spot formed on aninformation recording surface of the second optical disk comprises acentral spot portion, an intermediate spot portion and a peripheral spotportion, in the second light flux passing through the objective opticalelement.

In the meantime, when employing the structure in which the secondoptical path difference providing structure is arranged in theperipheral area of the objective optical element and the third lightflux passing through the peripheral area of the objective opticalelement forms a flare on an information recording surface of the thirdoptical disk, there occurs a problem that spherical aberrations of ahigh order are caused extremely, when the first light flux is used forrecording and/or reproducing for the first optical disk, and when awavelength of the first light flux is changed from the design wavelengthor a temperature is changed. Incidentally, the spherical aberrations ofa high order mentioned in this case mean spherical aberrations of fifthto ninth order. Therefore, by arranging so that the second optical pathdifference providing structure may comprise the second basic structure,the fourth basic structure or the fifth basic structure which will bedescribed later, it is possible to reduce the spherical aberration of ahigh order, even in the case where the first light flux is used for thefirst optical disk, a wavelength of the first light flux is changed fromthe design wavelength and a temperature is changed. What is especiallypreferable is to satisfy the following expressions (2) and (2′).Satisfying the following expression (2″) is more preferable.

δSAH/δλ≦0.010 (λrms/nm)  (2)

δSAH=√((δSA5)²+(δSA7)²+(δSA9)²)  (2′)

δSAH/δλ≦0.008 (λrms/nm)  (2″)

Where, δSA5 is a fifth order spherical aberration generated wheninformation is recorded and/or reproduced for the first optical diskusing a light flux with a wavelength λx which is shifted from a usingwavelength of 408 nm, and generated at a magnification, which makes athird order spherical aberration SA3 zero, of the light flux with thewavelength of λx for the objective optical element. δSA7 is a seventhorder spherical aberration generated when information is recorded and/orreproduced for the first optical disk using a light flux with thewavelength λx, and generated at a magnification, which makes a thirdorder spherical aberration SA3 zero, of the light flux with thewavelength of λx for the objective optical element. δSA9 is a ninthorder spherical aberration generated when information is recorded and/orreproduced for the first optical disk using a light flux with thewavelength λx, and generated at a magnification, which makes a thirdorder spherical aberration SA3 zero, of the light flux with thewavelength of λx for the objective optical element. δλ is an absolutevalue of a difference between 408 nm and λx nm. Incidentally, δλ that is10 nm or less is preferable.

Further, for reducing spherical aberrations of a high order, it ispreferable that according to a graph whose horizontal axis represents adistance from an optical axis in a direction of a radius of theobjective optical element, and whose vertical axis represents an opticalpath difference of the first light flux provided by the objectiveoptical element when the first light flux passes through the objectiveoptical element, the graph comprises a discontinuity at a wavelengthwhich is shifted by 5 nm from a designed wavelength of the firstwavelength of the objective optical element. Then it is also preferablethat the gap width of the optical path difference at the discontinuityis 0 or more, and 0.2 λ1 or less. When the gap width of the optical pathdifference is 0, it means that the graph has no discontinuous portion.

Further, it is preferable that the second optical path differenceproviding structure corrects spherochromatism (chromatic sphericalaberration) generated by slight fluctuation of a wavelength of the firstlight source or the second light source, for the first light flux andthe second light flux both passing through the second optical pathdifference providing structure. The slight fluctuation of a wavelengthmeans a fluctuation within ±10 nm. For example, when the first lightflux is changed in terms of a wavelength by 5 nm from wavelength λ1, itis preferable that the fluctuation of spherical aberration of the firstlight flux passing through a peripheral area is corrected by the secondoptical path difference providing structure, and an amount of change ofwavefront aberration on an information recording surface of the firstoptical disk is made to be 0.010 λ1 rms or more, and 0.095 λ1 rms orless. Further, when the second light flux is changed in terms of awavelength by 5 nm from wavelength λ2, it is preferable that thefluctuation of spherical aberration of the second light flux passing aperipheral area is corrected by the second optical path differenceproviding structure, and an amount of change of wave front aberration onan information recording surface of the second optical disk is made tobe 0.002 λ2 rms or more, and 0.03 λ2 rms or less. Owing to this, it ispossible to correct aberration caused by fluctuations of wavelengthresulting from manufacturing errors of a laser or an individualdifference of lasers.

It is preferable that the second optical path difference providingstructure further corrects a spherical aberration caused by temperaturechange in an objective optical element, for the first light flux and thesecond light flux both passing through the second optical pathdifference providing structure. For example, when a temperature of theobjective optical element is changed by 30° C., it is preferable thatthe fluctuation of spherical aberration of the first light flux or thesecond light flux passing through a peripheral area is corrected by thesecond optical path difference providing structure, and an amount ofchange of wavefront aberration on an information recording surface ofthe first optical disk is made to be 0.010 λ1 rms or more, and 0.095 λ1rms or less, and an amount of change of wavefront aberration on aninformation recording surface of the second optical disk is made to be0.002 λ2 rms or more, and 0.03 λ2 rms or less.

When the objective optical element comprises a most peripheral area, theobjective optical element converges the first light flux passing throughthe most peripheral area of the objective optical element so as torecord and/or reproduce information on an information recording surfaceof the first optical disk. Further, in the first light flux passingthrough the most peripheral area, it is preferable that its sphericalaberration is corrected in the case of recording and/or reproducinginformation for the first optical disk.

As a preferable embodiment, there is given an embodiment wherein asecond light flux passing through the most peripheral area is not usedfor recording and/or reproducing information for the second opticaldisk, and a third light flux passing through the most peripheral area isnot used for recording and/or reproducing information for the thirdoptical disk. It is preferable that the second light flux and the thirdlight flux both passing through the most peripheral area are arrangednot to contribute to forming of converged spot respectively on aninformation recording surface of the second optical disk and on aninformation recording surface of the third optical disk. In other words,when the objective optical element comprises the most peripheral area,it is preferable that the third light flux passing through the mostperipheral area of the objective optical element forms a flare on aninformation recording surface of the third optical disk. In other words,it is preferable that the third light flux passing through the mostperipheral area of the objective optical element forms a peripheral spotportion on an information recording surface of the third optical disk.Further, when the objective optical element comprises a most peripheralarea, it is preferable that the second light flux passing through themost peripheral area of the objective optical element forms a flare onan information recording surface of the second optical disk. In otherwords, it is preferable that the second light flux passing through themost peripheral area of the objective optical element forms a peripheralspot portion on an information recording surface of the second opticaldisk.

When the most peripheral area comprises the third optical pathdifference providing structure, it is also possible to arrange so thatthe third optical path difference providing structure correctsspherochromatism (chromatic spherical aberration) caused by slightfluctuations of a wavelength of the first light source, for the firstlight flux passing through the third optical path difference providingstructure. The slight fluctuation of a wavelength means a fluctuationwithin ±10 nm. For example, when the first light flux is changed interms of a wavelength by 5 nm from wavelength λ1, it is preferable thatthe fluctuation of spherical aberration of the first light flux passingthrough a most peripheral area is corrected by the third optical pathdifference providing structure, and an amount of change of wave frontaberration on an information recording surface of the first optical diskis made to be 0.010 λ1 rms or more, and 0.095 λ1 rms or less.

The third optical path difference providing structure corrects alsospherical aberration caused by temperature changes of the objectiveoptical element, for the first light flux passing through the thirdoptical path difference providing structure, which is preferable. Forexample, when a temperature of the objective optical element is changedby 30° C., the fluctuation of spherical aberration of the first lightflux passing through a most peripheral area is corrected by the thirdoptical path difference providing structure, and an amount of change ofwavefront aberration on an information recording surface of the firstoptical disk is made to be 0.010 λ1 rms or more, and 0.095 λ1 rms orless, which is preferable.

In the meantime, it is preferable that the first optical path differenceproviding structure is a structure comprising at least a first basicstructure.

The first basic structure is an optical path difference providingstructure which emits a first order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the first basicstructure; which emits a first order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the first basicstructure; and which emits a first order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the firstbasic structure. The first basic structure is preferably an optical pathdifference providing structure that causes diffraction anglesrespectively of the first light flux, the second light flux and thethird light flux all passing through the first basic structure to bedifferent from each other. It is further preferable that step amountalong the optical axis of the first basic structure provides an opticalpath difference of almost the same as large as the first wavelength withthe first light flux, provides an optical path difference of almost 0.6times as large as the second wavelength with the second light flux, andprovides an optical path difference of almost 0.5 times as large as thethird wavelength with the third light flux.

Further, for the purpose of correcting spherical aberration generated bytemperature changes, correcting chromatic aberration and/or causingparallel light or substantially parallel light to enter the objectiveoptical element in the course of conducting recording and/or reproducingfor all optical disks, a structure wherein the third basic structure orthe fifth basic structure is overlapped on the first basic structure mayalso be the first optical path difference providing structure.

The third basic structure is an optical path difference providingstructure which emits a second order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the third basicstructure; which emits a first order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the third basicstructure; and which emits a first order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the thirdbasic structure. The third basic structure is preferably an optical pathdifference providing structure that causes diffraction angle of thesecond light flux passing through the third basic structure to bedifferent from the diffraction angles of the first and the third lightfluxes passing through the third basic structure. It is furtherpreferable that step amount along the optical axis of the third basicstructure provides an optical path difference of almost 2 times as largeas the first wavelength with the first light flux, provides an opticalpath difference of almost 1.2 times as large as the second wavelengthwith the second light flux, and provides an optical path difference ofalmost the same as large as the third wavelength with the third lightflux. The fifth basic structure is an optical path difference providingstructure which emits a tenth order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the fifth basicstructure; which emits a sixth order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the fifth basicstructure; and which emits fifth order diffracted light fluxes each witha larger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the fifthbasic structure. It is further preferable that step amount along theoptical axis of the fifth basic structure provides an optical pathdifference of almost 10 times as large as the first wavelength with thefirst light flux, provides an optical path difference of almost 6 timesas large as the second wavelength with the second light flux, andprovides an optical path difference of almost 5 times as large as thethird wavelength with the third light flux.

The second optical path difference providing structure is of thestructure comprising at least a predetermined basic structure.

A predetermined basic structure in this case means an optical pathdifference providing structure which emits a x-th order diffracted lightflux with a larger light amount than any diffracted light fluxes withthe other diffraction order, when the first light flux passes throughthe predetermined basic structure; and which emits a y-th orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the second lightflux passes through the predetermined basic structure. Incidentally, xand y satisfy the following expression (8):

0.9·(x·λ1)/(n1−1)≦(y·λ2)/(n2−1)≦1.2·(x·λ1)/(n1−1)  (8)

Where, x represents an integer excluding 0, y represents an integerexcluding 0, n1 represents a refractive index of the objective opticalelement for the first light flux and n2 represents a refractive index ofthe objective optical element for the second light flux.

As a basic structure, satisfying above expression (8), there are given,for example, a second base structure, a fourth basic structure and thefifth basic structure previously described.

The second basic structure is an optical path difference providingstructure which emits a fifth order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the first light flux passes through the second basicstructure; which emits a third order diffracted light flux with a largerlight amount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the second basicstructure; and which emits third and second order diffracted lightfluxes each with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the third light flux passesthrough the second basic structure. It is preferable that the amount ofthe third order diffracted light flux is slightly larger than the amountof the second order diffracted light flux, in the third light flux. Itis preferable that step amount along the optical axis of the secondbasic structure provides an optical path difference of almost 5 times aslarge as the first wavelength with the first light flux, provides anoptical path difference of almost 3 times as large as the secondwavelength with the second light flux, and provides an optical pathdifference of almost 2.5 times as large as the third wavelength with thethird light flux. The fourth basic structure is an optical pathdifference providing structure which emits a third order diffractedlight flux with a larger light amount than any diffracted light fluxeswith the other diffraction order, when the first light flux passesthrough the fourth basic structure; which emits a second orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the second lightflux passes through the fourth basic structure; and which emits secondand first order diffracted light fluxes each with a larger light amountthan any diffracted light fluxes with the other diffraction order, whenthe third light flux passes through the fourth basic structure. It ispreferable that the amount of the second order diffracted light flux isslightly larger than the amount of the first order diffracted lightflux, in the third light flux. It is preferable that step amount alongthe optical axis of the fourth basic structure provides an optical pathdifference of almost 3 times as large as the first wavelength with thefirst light flux, provides an optical path difference of almost 1.9times as large as the second wavelength with the second light flux, andprovides an optical path difference of almost 1.6 times as large as thethird wavelength with the third light flux. Meanwhile, the second basicstructure, the fourth basic structure and the fifth basic structure(especially, the second basic structure and the fifth basic structure)have a function to make spherical aberration to be under-corrected whena temperature rises and wavelengths of the first light source, thesecond light source and the third light source are made longer.Accordingly, thereby, an over-corrected spherical aberration caused by adecline of the refractive index of plastic in the case of temperaturerise can be corrected, resulting in an excellent state with lessspherical aberration. Incidentally, it is possible to make a depth of astep to be smaller for the second basic structure than for the fifthbasic structure. Further, the second basic structure, the fourth basicstructure, or the fifth basic structure in the second optical pathdifference providing structure may be arranged on a base asphericsurface (base surface) which differs from the other basic structure. Itis preferable that the second basic structure, the fourth basicstructure, and the fifth basic structure in the second optical pathdifference providing structure may be arranged on a base asphericsurface (base surface) configured to provide the optical pathdifferences as described above for respective incident light fluxes andconfigured such that the second basic structure, the fourth basicstructure, and the fifth basic structure do not affect the direction ofthe incident light flux as well as possible. It is further preferablethat each of the second basic structure, the fourth basic structure, andthe fifth basic structure in the second optical path differenceproviding structure may be a structure such that the structure goes moreinside of the optical element in the direction of the optical axis atthe point on the optical element apart farther from the optical axis inthe perpendicular direction to the optical axis, then from a certainpoint, it goes more upper side of the optical element in the directionof the optical axis at the point on the optical element apart fartherfrom the optical axis in the perpendicular direction to the opticalaxis. In other words, each of the second basic structure, the fourthbasic structure, and the fifth basic structure in the second opticalpath difference providing structure may be a structure in which depth ofthe step becomes deeper as the point of the objective optical elementbecomes farther from the optical axis in the direction perpendicular tothe optical axis, and from the certain position, depth of the stepbecomes shallower as the point of the objective optical element becomesfarther from the optical axis in the direction perpendicular to theoptical axis.

Further, satisfying the following expression (8′) is preferable.

0.95·(x·λ1)/(n1−1)≦(y−λ2)/(n1−1)≦1.05·(x·λ1)/(n1−1)  (8′)

As a basic structure satisfying the aforesaid expression (8′), there isgiven, for example, the second basic structure or the fifth basicstructure. Hereupon, the fourth basic structure does not satisfy theexpression (8′).

Further, the second optical path difference providing structure ispreferably of the structure in which the predetermined basic structureand another basic structure are overlapped to each other, and anotherbasic structure stated above is either one of the third basic structure,the fourth basic structure previously described and a sixth basicstructure, which are preferable. The sixth basic structure is an opticalpath difference providing structure which emits a zero-th orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the first light fluxpasses through the sixth basic structure; which emits a first orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the second lightflux passes through the sixth basic structure; and which emits zero-thorder diffracted light flux each with a larger light amount than anydiffracted light fluxes with the other diffraction order, when the thirdlight flux passes through the sixth basic structure. The sixth basicstructure preferably comprises a plurality of small stepped structureseach having four steps. It is further preferable that step amount alongthe optical axis of one step of the small stepped structure provides anoptical path difference of almost 2 times as large as the firstwavelength with the first light flux, provides an optical pathdifference of almost 1.2 times as large as the second wavelength withthe second light flux, and provides an optical path difference of almostthe same as large as the third wavelength with the third light flux. Thewhole small stepped structure comprising four steps preferably providesan optical path difference of almost 8 times as large as the firstwavelength with the first light flux, provides an optical pathdifference of almost 5 times as large as the second wavelength with thesecond light flux, and provides an optical path difference of almost 4times as large as the third wavelength with the third light flux.

Further, the third basic structure may further be overlapped on thesecond optical path difference providing structure, for the purpose ofcorrecting chromatic aberration and/or causing parallel light orsubstantially parallel light to enter the objective optical element inthe course of conducting recording and/or reproducing for all opticaldisks.

Further, when an objective optical element is a plastic lens, it ispreferable to comprise a most peripheral area comprising the thirdoptical path difference providing structure. In this case, the thirdoptical path difference providing structure is a structure comprising atleast one of the first basic structure, the second basic structure, thethird basic structure, the fourth basic structure, the fifth basicstructure previously described and a eighth basic structure. The eighthbasic structure is an optical path difference providing structure whichemits a fourth order diffracted light flux with a larger light amountthan any diffracted light fluxes with the other diffraction order, whenthe first light flux passes through the eighth basic structure; whichemits third and second order diffracted light fluxes with a larger lightamount than any diffracted light fluxes with the other diffractionorder, when the second light flux passes through the eighth basicstructure; and which emits second order diffracted light flux with alarger light amount than any diffracted light fluxes with the otherdiffraction order, when the third light flux passes through the eighthbasic structure. It is further preferable that step amount along theoptical axis of the eighth basic structure provides an optical pathdifference of almost 4 times as large as the first wavelength with thefirst light flux, provides an optical path difference of almost 2.5times as large as the second wavelength with the second light flux, andprovides an optical path difference of almost 2 times as large as thethird wavelength with the third light flux. Incidentally, when thesecond optical path difference providing structure comprises the thirdbasic structure, the third optical path difference providing structurepreferably comprises the first basic structure, the second basicstructure, the fourth basic structure, the fifth basic structure or theeighth basic structure. On the other hand, when the second optical pathdifference providing structure comprises the fourth basic structure, thethird optical path difference providing structure preferably comprisesthe first basic structure, the second basic structure, the third basicstructure, the fifth basic structure or the eighth basic structure.Further, from the viewpoint of manufacturing, the second basic structureis preferable because it can be manufactured easily. Further, from theviewpoint of controlling a width of fluctuations in diffractionefficiency and in light utilization efficiency caused by temperaturechanges and wavelength changes to be smaller, the third basic structureand the fourth basic structure are preferable, but a transmittance isslightly lowered especially in the fourth basic structure. It istherefore possible to select the basic structure of the third opticalpath difference depending on the purpose.

When the objective optical element is a glass lens or a lens made ofathermal resin, it is preferable to have a most peripheral area being arefractive surface.

Each of the first optical path difference providing structure, thesecond optical path difference providing structure and the third opticalpath difference providing structure can be divided into plural areas,and each area can also be made of a different basic structure.Especially, each of the second optical path difference providingstructure and the third optical path difference providing structure canbe divided into plural areas, and each area can also be made of adifferent basic structure. For example, the second optical pathdifference providing structure can be divided into an inner side areacloser to an optical axis and an outer side area surrounding the innerside area, and the inner side area can be made of the structure whereinthe second basic structure and the third basic structure are overlappedto each other, while the outer side area can be made of the structurewherein the second basic structure, the third basic structure and aseventh basic structure which will be described later are overlapped toeach other.

From the viewpoint of realizing easy manufacture of a molding die and ofimproving transferring properties of a molding die, it is preferablethat a pitch width of a step is not too small. Therefore, whenring-shaped zones having a pitch width of 5 μm or less are generatedwhen designing an optical path difference providing structurerepresenting a basis by overlapping plural basic structures, it ispreferable to obtain a final optical path difference providing structureby eliminating ring-shaped zones having a pitch width of 5 μm or less.When ring-shaped zones with pitch width of 5 μm or less are convex, thering-shaped zones can be shaved off for eliminating the ring-shapedzones having a pitch width of 5 μm or less, while, when ring-shapedzones with pitch width of 5 μm or less are concave, it is possible tofill up the ring-shaped zones for eliminating.

Therefore, it is preferable that every pitch width of at least the firstoptical path difference providing structure is greater than 5 μm.Preferably, every pitch width in all of the first optical pathdifference providing structure, the second optical path differenceproviding structure and the third optical path difference providingstructure is greater than 5 μm.

Further, it is preferable that a step amount is not too large. When anamount of a step of ring-shaped zones equipped with an optical pathdifference providing structure representing a basis obtained byoverlapping plural basic structures is higher than a standard value, itis possible to reduce an excessive amount of a step without affectingoptical performances, by lowering an amount of a step of ring-shapedzones by 10·λB/(n−1) (μm). With respect to the standard value, it ispreferable to set 10·λB/(n−1) (μm) as a standard value, although anoptional value can be set as a standard value. The symbol λB representsa design wavelength (μm) of the first light flux, while, n represents arefractive index of an optical element for wavelength λB.

Meanwhile, the pitch width means a width of the ring-shaped zone in adirection perpendicular to the optical axis. The step amount means depthof the step of the ring-shaped zone in a direction of the optical axis.

From the viewpoint that less ring-shaped zones which are long and narroware preferable for manufacturing, it is preferable that a value of (stepamount/pitch width) is 1 or less for all ring-shaped zones of the firstoptical path difference providing structure, and it is more preferablethat the aforesaid value is 0.8 or less. What is more preferable is thata value of (step amount/pitch width) is 1 or less for all ring-shapedzones of all optical path difference providing structures, and it ismore preferable that the aforesaid value is 0.8 or less.

Here, NA1 represents the image side numerical aperture of the objectiveoptical element, necessary for reproducing and/or recording informationfor the first optical disk. NA2 (NA1≧NA2) represents that the image sidenumerical aperture of the objective optical element necessary forreproducing and/or recording for the information to the second opticaldisk. NA3 (NA2>NA3) represents that the image side numerical aperture ofthe objective optical element necessary for reproducing and/or recordinginformation for the third optical disk. It is preferable that NA1 is oneof: 0.8 or more, and 0.9 or less; and 0.55 or more, and 0.7 or less.Specifically, preferable NA1 is 0.85. It is preferable that NA2 is 0.55or more, and is 0.7 or less. Specifically, preferable NA2 is 0.60.Further, it is preferable that NA3 is 0.4 or more, and is 0.55 or less.Specifically, preferable NA3 is 0.45 or 0.53.

It is preferable that the border of the central area and the peripheralarea in the objective optical element is formed in a portioncorresponding to the range being 0.9·NA3 or more and being 1.2·NA3 orless (more preferably, 0.95·NA3 or more, and 1.15·NA3 or less) for thethird light flux. More preferably, the border of the central area andthe peripheral area of the objective optical element is formed in aportion corresponding to NA3. Further, it is preferable that the borderof the peripheral area and the most peripheral area of the objectiveoptical element is formed in a portion corresponding to the range being0.9·NA2 or more, and being 1.2·NA2 or less (more preferably, being0.95·NA2 or more, and being 1.15·NA2 or less) for the second light flux.More preferably, the border of the peripheral area and the mostperipheral area of the objective optical element is formed in a portioncorresponding to NA2. It is preferable that the border of the outside ofthe most peripheral area of the objective optical element is formed in aportion corresponding to the range being than 0.9·NA1 or more, and being1.2·NA1 or less (more preferably, being 0.95·NA1 or more, and being1.15·NA1 or less) for the first light flux. More preferably, the borderof the outside of the most peripheral area of the objective opticalelement is formed in a portion corresponding to NA1.

When the third light flux passing through the objective optical elementis converged on the information recording surface of the third opticaldisk, it is preferable that the spherical aberration has at least onediscontinuous portion. In that case, it is preferable that thediscontinuous portion exists in the range being 0.9·NA3 or more, andbeing 1.2·NA3 or less (more preferably, being 0.95·NA3 or more, andbeing 1.15·NA3 or less) for the third light flux. Further, also when thesecond light flux passing through the objective optical element isconverged on the information recording surface of the second opticaldisk, it is preferable that the spherical aberration has at least onediscontinuous portion. In that case, it is preferable that thediscontinuous portion exists in the range being 0.9·NA2 or more, andbeing 1.2·NA2 or less (more preferably, being 0.95·NA2 or more, andbeing 1.1·NA2 or less) for the second light flux.

Further, when the spherical aberration is continuous and does not havethe discontinuous portion, and when the third light flux passing throughthe objective optical element is converged on the information recordingsurface of the third optical disk, it is preferable that the absolutevalue of the vertical spherical aberration is 0.03 μm or more in NA2,and the absolute value of the vertical spherical aberration is 0.02 μmor less in NA3. More preferably, the absolute value of the verticalspherical aberration is 0.08 μm or more in NA2, and the absolute valueof the vertical spherical aberration is 0.01 μm or less in NA3. Further,when the second light flux passing through the objective optical elementis converged on the information recording surface of the second opticaldisk, it is preferable that the absolute value of the vertical sphericalaberration is 0.03 μm or more in NA1, and the absolute value of thevertical spherical aberration is 0.005 μm or less in NA2.

Further, because the diffraction efficiency depends on the depth (stepamount) of the ring-shaped zone in the diffractive structure, thediffraction efficiency of the central area for each wavelength can beappropriately set corresponding to the use of the optical pickupapparatus. For example, in the case of the optical pickup apparatus forrecording and reproducing information on the first optical disk, andonly for reproducing information on the second and the third opticaldisks, it is preferable that the diffraction efficiency of the centralarea and/or the peripheral area is defined with considering primarilythe diffraction efficiency for the first light flux. On the other hand,in the case of the optical pickup apparatus only for reproducinginformation on the fist optical disks and for recording and reproducinginformation on the second and third optical disks, it is preferable thatthe diffraction efficiency of the central area is defined withconsidering primarily the diffraction efficiency for the second andthird light fluxes and the peripheral area is defined with consideringprimarily the diffraction efficiency for the second light flux.

In any case, when the following conditional expression (14) issatisfied, the diffraction efficiency of the first light flux calculatedby the area weighted mean can be secured high.

η11≦η21  (14)

Where, η11 expresses is an efficiency of a light utilization of thefirst light flux in the central area, η21 expresses is an efficiency ofa light utilization of the first light flux in the peripheral area.Hereupon, when the efficiency of a light utilization of the central areais defined with considering primarily the light fluxes with the secondand the third wavelengths, the efficiency of a light utilization of thefirst light flux of the central area is decreased. However, in the casewhere the numerical aperture of the first optical disk is larger thanthe numerical aperture of the third optical disk, when considered on thewhole effective diameter of the first light flux, the decrease of theefficiency of a light utilization the central area does not give so muchlarge influence.

Further, it is preferable that the following expression (3) issatisfied:

η13≧40%  (3)

Where, η13 represents an efficiency of a light utilization for the thirdlight flux in a central area.

The light utilization efficiency mentioned in the present specificationcan be defined as follows. In the objective optical element in which thefirst optical path difference providing structure and the second opticalpath difference providing structure (and the third optical pathdifference providing structure) are formed, when a light flux enteringan area other than a target for measurement is intercepted, A representsa light amount in an airy disc of a converged spot formed on aninformation recording surface of an optical disk corresponding to thelight flux used for measurement. While, in the objective optical elementformed of the same material to the above objective optical element andhaving the same focal length, axial thickness, numerical aperture, andwavefront wavelength, in which none of the first optical path differenceproviding structure, the second optical path difference providingstructure, and the third optical path difference providing structure areformed, when a light flux entering an area other than a target formeasurement is intercepted, B represents a light amount in an airy discof a converged spot formed on an information recording surface of anoptical disk corresponding to the light flux used for measurement. Thelight utilization efficiency is to be calculated based on A/B.Meanwhile, the airy disc mentioned in this case means a circle withradius r′ whose center is on an optical axis of the converged spot. Theradius of the circle is indicated by r′=0.61·λ/NA.

For recording and/or reproducing information for the first optical diskand the second optical disk properly while improving light utilizationefficiency in the case of recording and/or reproducing information forthe third optical disk, it is preferable that a blaze wavelength is madeto be a wavelength between the first wavelength and the secondwavelength. It is more preferable that the blaze wavelength is not lessthan 405 nm and is not more than 600 nm.

In the third light flux passing through the first optical pathdifference providing structure, it is preferable that a differencebetween a light amount for diffracted light having a diffraction ordernumber that makes a light amount to be a maximum and a light amount fordiffracted light having a diffraction order number that makes a lightamount to be second largest, namely, a difference between a light amountfor diffracted light forming the first best focus and a light amount fordiffracted light forming the second best focus is 15% or more. Thedifference of 30% or more is more preferable.

The first light flux, the second light flux, and the third light fluxmay either enter the objective optical element as parallel light fluxes,or enter the objective optical element as divergent light fluxes orconvergent light fluxes. Preferably, magnification m1 of the first lightflux entering into the objective optical element satisfies the followingexpression (4).

−0.02<m1<0.02  (4)

While, magnification m1 of the first light flux entering into theobjective optical element as a divergent light flux satisfies thefollowing expression (4′).

−0.10<m1<0.00  (4′)

When the second light flux enters into the objective optical element asa parallel light flux or as a substantially parallel light flux, it ispreferable that magnification m2 of the second light flux entering intothe objective optical element satisfies the following expression (5).

−0.02<m2<0.02  (5)

On the other hand, when the second light flux enters into the objectiveoptical element as a divergent light flux, it is preferable thatmagnification m2 of the second light flux entering into the objectiveoptical element satisfies the following expression (5′).

−0.10<m2<0.00  (5′)

When the third light flux enters into the objective optical element as aparallel light flux or as a substantially parallel light flux, it ispreferable that magnification m3 of the third light flux entering intothe objective optical element satisfies the following expression (6).

−0.02<m3<0.02  (6)

When the aforesaid expression (6) is satisfied, namely, when the thirdlight flux enters into the objective optical element as a parallel orsubstantially parallel light flux, the objective optical elementpreferably comprises a fourth optical path difference providingstructure. When providing the fourth optical path difference providingstructure on the objective optical element, the fourth optical pathdifference providing structure preferably is provided on an opticalsurface which is different from the optical surface on which the firstoptical path difference providing structure and the second optical pathdifference providing structure are provided. Further, the fourth opticalpath difference providing structure is preferably provided on thesurface of the objective optical element closer to the optical diskside. The fourth optical path difference providing structure ispreferably of the structure to correct chromatic spherical aberration.Further, the fourth optical path difference providing structure ispreferably of the structure to comprise any one of the second basicstructure, the third basic structure, the fifth basic structure and thesixth basic structure. Incidentally, when the fourth optical pathdifference providing structure comprises the third basic structure, thefourth optical path difference providing structure may have a functionto correct spherical aberration caused by a difference between thicknesst1 of a protective substrate of the first optical disk and thickness t2of a protective substrate of the second optical disk for the first lightflux and the second light flux, and this function can be shared by thesecond optical path difference providing structure and the fourthoptical path difference providing structure, which is preferable.

When the aforesaid expression (6) is satisfied, a basic structure havingthe function to correct chromatic aberration may also be overlapped onat least one of the first optical path difference providing structure,the second optical path difference providing structure and the thirdoptical path difference providing structure (preferably, two of them,more preferably, all of them), without providing the fourth optical pathdifference providing structure. For example, it is also possible toemploy the structure wherein either one of the second basic structure,the third basic structure, the fifth basic structure and the sixth basicstructure is overlapped on at least one of the first optical pathdifference providing structure, the second optical path differenceproviding structure and the third optical path difference providingstructure. Especially, when the expressions (4), (5), and (6) aresatisfied, especially when all of the first light flux, second lightflux, and third light flux enters into the objective optical element asparallel light fluxes and the fourth optical path difference providingstructure are not provided on the objective optical element, the thirdbasic structure is preferably overlapped to the first optical pathdifference providing structure.

On the other hand, when the third light flux enters into the objectiveoptical element as a divergent light flux, it is preferable thatmagnification m3 of the third light flux entering into the objectiveoptical element satisfies the following expression (7).

−0.10<m3<0.00  (7)

When the objective optical element comprises a plastic lens, it ispreferable to keep excellent temperature characteristic. In this case,it is preferable that the expression (16) is satisfied. Further it ispreferable to keep balance between a wavelength characteristic and atemperature characteristic. To satisfy these characteristics, followingconditional expressions (16) and (17) are preferably satisfied:

+0.0005 (WFEλrms/(° C.·mm))≦δSAT1/f′≦+0.0020 (WFEλrms/(° C.·mm))  (16)

−0.00020 (WFEλrms/(nm·mm))≦δSAλ/f′≦−0.00002 (WFEλrms/(nm·mm))  (17)

where, δSAT1 represents δSA3/δT (rate of change of third order sphericalaberration caused by temperature change) of the objective opticalelement in the case of recording/reproducing information for an opticaldisk in a using wavelength (assuming no wavelength fluctuation caused bytemperature change, in this case). The using wavelength means awavelength of a light source used in an optical pickup apparatuscomprising an objective optical element. Preferably, the usingwavelength is a wavelength being 400 nm or more, and 415 nm or less withwhich information can be recorded and/or reproduced for an optical diskthrough an objective optical element. When the using wavelength cannotbe established as in the foregoing, 408 nm may be used as the usingwavelength to obtain δSAT1 of the objective optical element as well asδSAT2 and δSAT3 which will be described later. Meantime, WFE shows thatthird order spherical aberration is expressed by wavefront aberration.Further, δSAλ represents δSA3/δλ (rate of change of third orderspherical aberration caused by wavelength change) of the objectiveoptical element in the case of recording and/or reproducing informationfor an optical disk in the using wavelength. Incidentally, it ispreferable that an ambient temperature is a room temperature. The roomtemperature is 10° C. or more, and 40° C. or less, and it preferably is25° C. The symbol f′ represents a focal length of the objective opticalelement in a light flux with wavelength λ1 (preferably, wavelength 408nm).

In further description, it is preferable to consider changes inwavelength of a light source caused by temperature changes, in additionto changes in spherical aberration of the objective optical elementcaused by temperature changes. What is preferable is to satisfy thefollowing conditional expression (18):

0 (WFEλrms/(° C.·mm))≦δSAT2/f′≦+0.0020 (WFEλrms/(° C.·mm))  (18)

where, δSAT2 represents δSA3/δT of the objective optical element in thecase of recording and/or reproducing information for an optical disk ina light source wherein wavelength fluctuation caused by temperaturechanges is 0.05 nm/° C.

It is more preferable that the following conditional expression (18′) issatisfied.

0 (WFEλrms/(° C.·mm))≦δSAT2/f′≦+0.0015 (WFEλrms/(° C.·mm))  (18′)

When a light converging optical system of the optical pickup apparatuscomprises a coupling lens such as a collimator lens, and the couplinglens is a plastic lens, satisfying the following conditional expression(19) is preferable:

0 (WFEλrms/(° C.·mm))≦δSAT3/f′≦+0.0015 (WFEλrms/(° C.·mm))  (19)

where, δSAT3 represents δSA3/δT of the total optical system including acoupling lens and an objective optical element, in the case of recordingand/or reproducing information for a high density optical disk in alight source wherein wavelength fluctuation caused by temperaturechanges is 0.05 nm/° C.

A working distance (WD) of an objective optical element in the case ofusing a third optical disk is preferably 0.15 mm or more, and 1.5 mm orless. It is more preferable to be 0.2 mm or more, and 0.4 mm or less.Next, WD of an objective optical element in the case of using a secondoptical disk is preferably 0.3 mm or more, and 0.7 mm or less. Further,WD of an objective optical element in the case of using a first opticaldisk is preferably 0.4 mm or more, and 0.9 mm or less. For t1<t2, it ispreferable to be 0.6 mm or more, and 0.9 mm or less.

When the objective optical element is a single lens, the thinner axialthickness of the lens is more preferable for lengthening the above WD.On the other, however, if the lens is made to be too thin, an influenceof the temperature change on the plastic lens grows too great, which isnot preferable. Thus, satisfying the following conditional expression(15) is preferable.

1≦T/f≦1.13  (15)

Where, T (mm) represents an axial thickness of the objective opticalelement and f (mm) represents a focal length of the objective opticalelement in the third light flux. It is preferable that an axialthickness of the objective optical element is 2.31 mm or more, and 2.61mm or less.

An incident pupil diameter of the objective optical element ispreferably 2.8 mm or more, and 4.5 mm or less when a first optical diskis used.

From the viewpoint of reducing, as far as possible, an influence exertedon a sensor for tracking by unwanted light passing through a centralarea, in the case of conducting tracking for a third optical disk suchas CD, the following conditional expression (1″) is preferablysatisfied.

0.02≦L/f<0.05  (1″)

Satisfying the following conditional expression (1′″) is morepreferable.

0.032<L/f<0.05  (1′″)

However, if a distance between a first best focus and a second bestfocus is extended as stated above, there comes into being a possibilitythat a third light flux passing through the peripheral area also entersa light receiving element to have an influence on accuracy of recordingand/or reproducing in the case of recording and/or reproducing for athird optical disk. When satisfying the aforesaid conditional expression(1″) for reducing the possibility to exert the influence, it ispreferable 1) to provide a structure for outreaching a third light fluxas a flare as far as possible on a peripheral area and/or on a mostperipheral area, or 2) to provide an aperture restricting element.

First, 1) providing the structure for outreaching a third light flux asa flare as far as possible on a peripheral area and/or on a mostperipheral area, will be explained.

A preferable state of the third light flux to pass through a peripheralarea and/or a most peripheral area in recording and/or reproducing for athird optical disc will be explained first. When recording and/orreproducing of the third optical disk is assumed, in the verticalspherical aberration diagram whose vertical axis is a height from theoptical axis along the direction perpendicular to the optical axis,whose horizontal axis is a defocus amount, it can be regarded as a morepreferable situation when an absolute value of a difference between adefocus amount of the third light flux passing through the central areaof the objective optical element and a defocus amount of the third lightflux passing through the peripheral area and/or the most peripheral areais larger. Therefore, when satisfying the aforesaid conditionalexpression (1″), and when a minimum value of an absolute value of adifference between a defocus amount of the third light flux passingthrough the central area of the objective optical element and a defocusamount of the third light flux passing through the peripheral areaand/or the most peripheral area is 10 μm or less in the verticalspherical aberration diagram in which recording and/or reproducing forthe third optical disk is assumed, it is preferable to take thefollowing action. The action is to provide an optical path differenceproviding structure for outreaching the third light flux as a flare asfar as possible on the portion having a height from the optical axis bywhich the difference of defocus amount is 10 μm or less, so that theminimum value of the difference between the defocus amount of the thirdlight flux passing through the central area of the objective opticalelement and the defocus amount of the third light flux passing throughthe peripheral area and/or the most peripheral area may become largerthan 10 μm, which is preferable. Namely, in the vertical sphericalaberration diagram in the case of satisfying the aforesaid conditionalexpression (1″), it is preferable that the minimum value for theabsolute value of the difference between the defocus amount of the thirdlight flux passing through the central area of the objective opticalelement (which may also be an area that is not more than a necessarynumerical aperture of the third optical disk) and the defocus amount ofthe third light flux passing through the peripheral area and/or the mostperipheral area (which may also be an area that is not less than anecessary numerical aperture of the third optical disk) is larger than10 μm. The minimum value exceeding 15 μm is more preferable.

Meanwhile, when satisfying conditional expression (1″), it is preferablethat the second optical path difference providing structure and/or thethird optical path difference providing structure comprises, on at leaston part of its area, the seventh basic structure, as a structure foroutreaching the third light flux as a flare as far as possible, inaddition to other basic structures. The seventh basic structure is anoptical path difference providing structure which emits a zero-th orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the first light fluxpasses through the seventh basic structure; which emits a zero-th orderdiffracted light flux with a larger light amount than any diffractedlight fluxes with the other diffraction order, when the second lightflux passes through the seventh basic structure; and which emits a firstorder diffracted light flux with a larger light amount than anydiffracted light fluxes with the other diffraction order, when the thirdlight flux passes through the seventh basic structure. It is furtherpreferable that step amount along the optical axis of the seventh basicstructure provides an optical path difference of almost 5 times as largeas the first wavelength with the first light flux, provides an opticalpath difference of almost 3 times as large as the second wavelength withthe second light flux, and provides an optical path difference of almost2.5 times as large as the third wavelength with the third light flux.The seventh basic structure preferably comprises a binary shaperepeating one step concavity and convexity. As an example, there isgiven the second optical path difference providing structure whoseinside area closer to an optical axis has the structure in which thesecond basic structure and the third basic structure are overlapped toeach other and whose outside area farther than the inside area from anoptical axis has the structure in which the second basic structure, thethird basic structure and the seventh basic structure are overlapped toeach other. In particular, it is preferable to provide the seventh basicstructure on a portion of the height from an optical axis described inthe previous paragraph. In many cases, it is preferable to provide theseventh basic structure on the side of the peripheral area that isfarther from the optical axis, and to provide the seventh basicstructure on the side of the most peripheral area that is closer to theoptical axis.

Next, the preferable state of the second light flux that passes throughthe most peripheral area in the course of recording and/or reproducingfor the second optical disk, will be explained. When recording and/orreproducing of the second optical disk is assumed, in the verticalspherical aberration diagram, it can be regarded as a more preferablesituation when an absolute value of a difference between a defocusamount of the second light flux passing through the central area and theperipheral area of the objective optical element and a defocus amount ofthe second light flux passing through the most peripheral area islarger. Therefore, when satisfying the aforesaid conditional expression(1″), and when a minimum value of an absolute value of a differencebetween a defocus amount of the second light flux passing through thecentral area and the peripheral area of the objective optical elementand a defocus amount of the second light flux passing through the mostperipheral area is 10 μm or less, in the vertical spherical aberrationdiagram in which recording and/or reproducing for the second opticaldisk is assumed, it is preferable to take the following action. Theaction is to provide an optical path difference providing structure foroutreaching the second light flux as a flare as far as possible on theportion having a height from the optical axis by which the difference ofdefocus amount is 10 μm or less, so that the minimum value of thedifference between the defocus amount of the second light flux passingthrough the central area and the peripheral area of the objectiveoptical element and the defocus amount of the second light flux passingthrough the most peripheral area may become larger than 10 μm, which ispreferable. Namely, in the vertical spherical aberration diagram in thecase of satisfying the aforesaid conditional expression (1″), it ispreferable that the minimum value for the absolute value of thedifference between the defocus amount of the second light flux passingthrough the central area and the peripheral area of the objectiveoptical element (which may also be an area that is not less than anecessary numerical aperture of the second optical disk) and the defocusamount of the second light flux passing through the most peripheral area(which may also be an area that is not less than a necessary numericalaperture of the second optical disk) is larger than 10 μm. The minimumvalue exceeding 15 μm is more preferable.

Incidentally, when satisfying the conditional expression (1″), it ispreferable to provide a structure for outreaching a second light flux asa flare as far as possible on the third optical path differenceproviding structure. For example, the third optical path differenceproviding structure can comprise, on at least a part of its area, athird basic structure as a structure for outreaching the second lightflux greatly as a flare. Meanwhile, it is difficult to declare the beststructure for outreaching the second light flux greatly as a flare,because of relationship with other optical path difference providingstructures. As an example, there is given the third optical pathdifference providing structure whose inside area closer to an opticalaxis has the structure that is composed only of the third basicstructure and whose outside area farther than the inside area from theoptical axis has the structure that is composed only of the second basicstructure. Further, in this example, it is also possible to divide theinside area of the third optical path difference providing structureinto two areas and thereby to make an inside-inside area of the thirdoptical path difference providing structure closer to an optical axis tohave the structure that is composed of third basic structure and theseventh basic structure and to make an outside-inside area that isfarther from the optical axis than the inside-inside area to have thestructure that is composed only of third basic structure. As otherexample, it is also possible to divide the third optical path differenceproviding structure into two areas, inside area and outside area, andthereby to make an inside area of the third optical path differenceproviding structure closer to an optical axis to have the structure inwhich the seventh basic structure and the second basic structure areoverlapped with each other and to make an outside area of the thirdoptical path difference providing structure farther from the opticalaxis than the inside area to have the structure composed only of thesecond basic structure.

Next, providing an opening limiting element of 2) will be explained.When providing the seventh basic structure to the second optical pathdifference providing structure and/or the third optical path differenceproviding structure as stated above, there is a possibility that +firstorder diffracted light and/or −first order diffracted light of the thirdlight flux generated by the seventh basic structure enters alight-receiving element, depending on the optical design. To avoid thispossibility, it is preferable to provide an opening limiting element. Inthis case, it is not necessary to provide the seventh basic structure tothe second optical path difference providing structure and/or the thirdoptical path difference providing structure.

It is preferable that the opening limiting element is arranged on thepoint closer to the first light source, the second light source, and thethird light source than the objective optical element in a commonoptical path of the first, second and third light fluxes. Further, theopening limiting element comprises a first area closest to an opticalaxis, and a second area arranged farther position from the optical axisthan the first area. The first area transmits the first light flux, thesecond light flux, and the third light flux. The second area transmitsthe first light flux and the second light flux and the third light fluxare not converged at a light converging position of the third light fluxwhich passes through the first area of the opening limiting element andthe objective optical element. As the second area is in two typesincluding a type which does not converge light at the convergingposition of the third light flux which passes through the first area bytransmitting the first light flux and the second light flux andpreventing the third light flux from transmitting as a dichroic filter;and a type which does not converge light at the conversing position ofthe third light flux which passes through the first area by transmittingthe first light flux and the second light flux and making the thirdlight flux flare, as a diffractive optical element. The third lightfluxes which passes through the first area enters into the central areaof the objective optical element. As a substantial example of theopening limiting element of this kind, a dichroic filter and adiffractive optical element are preferably used, and in particular, thedichroic filter wherein blue light, red light and infrared light aretransmitted in the area close to the optical axis, and blue light andred light are transmitted and infrared light is not transmitted in thearea that is away from the optical axis, is used favorably.

Further, in many cases, an optical pickup apparatus comprises a λ/4wavelength plate for converting a linear polarized light to a circularpolarized light or for converting a circular polarized light to a linearpolarized light. The opening limiting element may also be united withthis λ/4 wavelength plate.

As preferable examples of the λ/4 wavelength plate, there are givenroughly the following three types, to which, however, the examples areis not limited thereto. As a first type of the λ/4 wavelength plate,there is given a λ/4 wavelength plate having a high polymer liquidcrystal layer which is hardened liquid crystal monomer. For example,there is given a phase plate equipped with a first organic thin filmlayer and a second organic thin film layer. In the phase plate, thefirst organic thin film layer has a retardation value of ½ wavelengthfor light of a certain zone (for example, a visible zone), and thesecond organic thin film layer has a retardation value of ¼ wavelengthfor light of the same zone, and the first and second organic thin filmlayers are overlapped each other so that an optical axis of the firstorganic thin film layer and that of the second organic thin film layermay intersect at a prescribed angle respectively. These first and secondorganic thin film layers are high polymer liquid crystal layers. To bespecific, for example, contents described in Japanese Patent PublicationOpen to Public Inspection No. 2004-198942 can be applied to the λ/4wavelength plate of the first type.

As the λ/4 wavelength plate of the second type, there is given a λ/4wavelength plate having structural birefringence. For example, there isgiven one in which two types of media each having a different refractiveindex are arranged one after the other at a microscopic cycle length(for example, 100-300 nm), and a refractive index cyclic structureshowing structural birefringence is provided, and a phase difference isproduced by the structural birefringence. Further, as another example,there is given one in which plural wavelength plate elements having astructure of irregular cycle of λ/2<P<λ (P represents structural cycle(μm) and λ represents a wavelength (μm)) are used, and structuraldimensions of respective wavelength plate elements are determined sothat light transmittance may become high, and are combined. To bespecific concerning the latter example, for example, contents describedin Japanese Patent Publication Open to Public Inspection No. 2006-139263can be applied to the λ/4 wavelength plate of the second type.

As the λ/4 wavelength plate of the third type, there is given a λ/4wavelength plate in which plural high polymer film layers for convertinga circular polarized light to a linear polarized light or converting alinear polarized light to a circular polarized light for a specificwavelength zone are layered. To be specific, for example, contentsdescribed in EP1134068A can be applied to the λ/4 wavelength plate ofthe third type.

The optical information recording and reproducing apparatus comprisesthe optical disk drive apparatus having the above described opticalpickup apparatus.

Herein, the optical disk drive apparatus installed in the opticalinformation recording and reproducing apparatus will be described. Thereis provided the optical disk drive apparatus employing a system oftaking out only a tray which can hold an optical disk with the opticaldisk being, from the main body of the optical information recording andreproducing apparatus in which optical pickup apparatus is housed; and asystem of taking out the main body of the optical disk drive apparatusin which the optical pickup apparatus is housed.

The optical information recording and reproducing apparatus using eachof the above described systems, is generally provided with the followingcomponent members: an optical pickup apparatus housed in a housing; adrive source of the optical pickup apparatus such as seek-motor by whichthe optical pickup apparatus is moved toward the inner periphery orouter periphery of the optical disk for each housing; traveling meanshaving a guide rail for guiding the optical pickup apparatus toward theinner periphery or outer periphery of the optical disk; and a spindlemotor for rotation driving of the optical disk. However, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art.

The optical information recording and reproducing apparatus employingthe former system is provide with, other than these component members, atray which can hold the optical disk with the optical disk being mountedthereon, and a loading mechanism for slidably moving the tray. Theoptical information recording and reproducing apparatus employing thelatter system does not include the tray and loading mechanism, and it ispreferable that each component member is provided in the drawercorresponding to chassis which can be taken out outside.

The invention makes it possible to provide an optical pickup apparatus,an objective optical element and an optical information recording and/orreproducing apparatus in which an appropriated flare can be generated inthe course of recording and/or reproducing CD and DVD even when a singlelens is used as an objective optical element, and recording and/orreproducing of information can be conducted properly for three types ofdiscs such as high density optical disk, DVD and CD, whereinsimplification of their structures and low cost of them can be realized.In addition, it is possible to provide an optical pickup apparatus, anobjective optical element and an optical information recording and/orreproducing apparatus wherein light utilization efficiency can beenhanced and sufficient light amount can be secured for all of the threedifferent optical disks.

EXAMPLES

An embodiment of the invention will be explained as follows, referringto the drawings. FIG. 4 is a diagram schematically showing the structureof optical pickup apparatus PU1 in the present embodiment which canrecord and/or reproduce information properly for BD, DVD and CD eachbeing different from others. The optical pickup apparatus PU1 of thiskind can be installed in an optical information recording and/orreproducing apparatus. In this case, BD represents a first optical disk,DVD represents a second optical disk and CD represents a third opticaldisk. The invention is not limited to the present embodiment.

The optical pickup apparatus PU1 comprises light source package LDP inwhich violet semiconductor laser LD1 (first light source) that emits aviolet laser light flux (first light flux) with wavelength 408 nmemitted in the case of recording and/or reproducing information for BD,and red semiconductor laser LD2 (second light source) that emits a laserlight flux (second light flux) with wavelength 658 nm emitted in thecase of recording and/or reproducing information for DVD are packed inone package; hologram-laser HL composed of infrared semiconductor laserLD3 (third light source) emitting a laser light flux (third light flux)with wavelength 785 nm emitted in the case of recording and/orreproducing information for CD and of photodetector PD2 that receivesreflected light flux coming from information recording surface RL3 ofCD; common photodetector PD1 for BD and DVD (Plural light-receivingsections respectively for BD and DVD may also be provided); singleobjective lens (objective optical element) OL of a one lens group typeand is made of polyolefin-based plastic; biaxial actuator AC1, uniaxialactuator AC2; beam expander EXP that is arranged in a common opticalpath through which the first—third light fluxes pass, and is composed offirst lens L1 movable in the optical axis direction by uniaxial actuatorAC2 and second lens L2; first polarized light beam splitter BS1; secondpolarized light beam splitter BS2; ¼ wavelength plate QWP; sensor lensSEN; first collimator COL1 that is arranged in an optical path throughwhich the first light flux and the second light flux pass, and convertsthe first light flux and the second light flux into parallel lightfluxes; and second collimator COL2 that is arranged in an optical paththrough which the third light flux only passes, and converts the thirdlight flux into collimated light flux. Incidentally, violet SHG lasercan also be used in addition to the aforesaid violet semiconductor laserLD1, as a light source for BD. Meanwhile, it is preferable that acoupling lens through which the first light flux passes, namely thefirst collimator COL1 has, on its optical surface, the structure havinga function to correct chromatic aberration, such as an optical pathdifference providing structure having the second basic structure.

On an optical surface on the light source side of the objective lens OLin the present embodiment, there are formed central area CN including anoptical axis, peripheral area MD arranged to surround the central areaand the most peripheral area OT arranged to surround further theperipheral area, with forming concentric circles whose center is on theoptical axis, as shown in FIGS. 1 and 5. Meantime, a ratio of an area ofeach of the central area, the peripheral area and the most peripheralarea in FIGS. 1 and 5 is not shown accurately.

In optical pickup apparatus PU1, when recording and/or reproducinginformation for BD, a position of the first lens L1 in the optical axisdirection is adjusted by uniaxial actuator AC2 so that the first lightflux may be emitted from beam expander EXP under the state of a parallellight flux, then the violet semiconductor laser LD1 emits light. Asshown by a light path drawn with solid lines in FIG. 4, the divergentlight flux emitted from the violet semiconductor laser LD1 is convertedinto a parallel light flux by collimator COL1 after being reflected byfirst polarized beam splitter BS1. The parallel light flux is expandedin terms of a diameter by beam expander EXP and passes through ¼wavelength plate QWP to be regulated in terms of a light flux diameterby an unillustrated diaphragm. The regulated light flux enters intoobjective lens OL under the state of a parallel light flux to become aspot formed on information recording surface RL1 through protectivesubstrate PL1 of BD. A converged spot is formed on information recordingsurface RL1 of BD by the light flux which passes through central areaCN, peripheral area MD, most peripheral area OT and an optical surfaceon the optical disk side, of the objective lens. Focusing and trackingfor the objective lens OL are carried out by biaxial actuator AC1.

The light flux on information recording surface RL1 is reflected andmodulated by information pits on information recording surface RL1. Thereflected light flux is converted into a converged light flux by thefirst collimator COL1, after being transmitted again through objectivelens OL, ¼ wavelength plate QWP, beam expander EXP and second polarizedbeam splitter BS2. The converged light flux is given astigmatism bysensor lens SEN after being transmitted through the first polarized beamsplitter BS1, and is converged on a light-receiving surface ofphotodetector PD1. Thus, it is possible to read information recorded onBD by using output signals coming from the photodetector PD1.

Further, in the optical pickup apparatus PU1, when conducting recordingand/or reproducing of information for DVD, a position of the first lensL1 in the optical axis direction is adjusted by uniaxial actuator AC2 sothat second light flux may be emitted from beam expander EXP under thestate of a collimated light flux, then the red semiconductor laser LD2emits light. As shown by a light path drawn with dotted lines in FIG. 4,the divergent light flux emitted from the red semiconductor laser LD2 isconverted into a parallel light flux by collimator COL1 after beingreflected by first polarized beam splitter BS1. The parallel light fluxis expanded in terms of a diameter by beam expander EXP and passesthrough ¼ wavelength plate QWP to be regulated in terms of a light fluxdiameter by an unillustrated diaphragm. The light flux enters intoobjective lens OL under the state of a parallel light to become a spotformed on information recording surface RL2 through protective substratePL2 of DVD. A converged spot, namely, a spot central portion is formedon information recording surface RL2 of DVD by the light flux whichpasses through central area CN, peripheral area MD and an opticalsurface on the optical disk side, of the objective lens OL. A light fluxwhich passes through the most peripheral area OT is made to be a flareto form a peripheral spot portion. Focusing and tracking for theobjective lens OL are carried out by biaxial actuator AC1 arranged onthe circumference of the objective lens OL.

The light flux on information recording surface RL2 is reflected andmodulated by information pits on information recording surface RL2 andis converted into a converged light flux by the first collimator COL1,after being transmitted again through objective lens OL, ¼ wavelengthplate QWP, beam expander EXP and second polarized beam splitter BS2. Theconverged light flux is given astigmatism by sensor lens SEN after beingtransmitted through the first polarized beam splitter BS1, and isconverged on a light-receiving surface of photodetector PD1. Thus, it ispossible to read information recorded on DVD by using output signalscoming from the photodetector PD1.

Further, in the optical pickup apparatus PU1, when conducting recordingand/or reproducing of information for CD, a position of the first lensL1 in the optical axis direction is adjusted by uniaxial actuator AC2 sothat third light flux may be emitted from beam expander EXP under thestate of a slightly converged light flux (Example 1) or under the stateof a parallel light flux (Example 2), then the infrared semiconductorlaser LD3 emits light. As shown by a light path drawn with one-dot chainlines in FIG. 4, the divergent light flux emitted from the infraredsemiconductor laser LD3 is converted into a collimated light flux bysecond collimator COL2. After that, the collimated light flux isreflected by second polarized light beam splitter BS2, to be changed toslightly divergent light flux by beam expander EXP or to be expanded interms of a diameter while keeping its parallel light flux. Then, thelight flux passes through ¼ wavelength plate QWP to become a spot formedon information recording surface RL3 through protective substrate PL3 ofCD after entering objective lens OL under the state of a slightly finitedivergent light or of a parallel light. A converged spot, namely, a spotcentral portion is formed on information recording surface RL3 of CD bythe light flux which passes through central area CN and an opticalsurface on the optical disk side of the objective lens OL. A light fluxwhich passes through the most peripheral area OT and peripheral area MDis made to be a flare to form a peripheral spot portion. Focusing andtracking for the objective lens OL are carried out by biaxial actuatorAC1 arranged on the circumference of the objective lens OL.

A reflected light flux modulated by information pits on informationrecording surface RL3 is transmitted again through objective lens OL, ¼wavelength plate QWP and beam expander EXP, and is reflected by thesecond polarized beam splitter BS2, to be converted into a convergedlight flux by the collimator COL2. After that, the converged light fluxis converged on photodetector PD2. Thus, information recorded on CD canbe read by the use of output signals of the photodetector PD2.

When the first light flux emitted from violet semiconductor laser LD1enters objective optical element OL, the first optical path differenceproviding structure of the central area, the second optical pathdifference providing structure of the peripheral area and third opticalpath difference providing structure of the most peripheral area cancorrect properly spherical aberration of the first light flux, and canrecord and/or reproduce information properly for BD with a protectivesubstrate of thickness t1. When the second light flux emitted from redsemiconductor laser LD2 enters objective optical element OBJ, the firstoptical path difference providing structure of the central area and thesecond optical path difference providing structure of the peripheralarea can correct properly spherical aberration of the second light fluxgenerated by a difference of protective substrate thickness between BDand DVD and by a difference of wavelength between the first light fluxand the second light flux, and can record and/or reproduce informationproperly for DVD with a protective substrate of thickness t2, becausethe most peripheral area causes the second light flux to be a flare onan information recording surface of DVD. When the third light fluxemitted from infrared semiconductor laser LD3 enters objective opticalelement OL, the first optical path difference providing structure of thecentral area can correct properly spherical aberration of the thirdlight flux generated by a difference of protective substrate thicknessbetween BD and CD and by a difference of wavelength between the firstlight flux and the third light flux, and can record and/or reproduceinformation properly for CD with protective substrate of thickness t3,because the second optical path difference providing structure of theperipheral area and the third optical path difference providingstructure of the most peripheral area make the third light flux to be aflare on an information recording surface of CD. Further, since thefirst optical path difference providing structure of the central areamakes the diffraction efficiency of the third light flux used forrecording and reproducing to be an excellent one, a light amount of thethird light flux which is sufficient for recording and reproducing canbe obtained. In addition, the second optical path difference providingstructure of the peripheral area can correct spheromatism (chromaticspherical aberration) when the wavelength is deviated from the standardwavelength for the first and second light fluxes for reasons such asmanufacturing errors of lasers, and can correct spherical aberrationgenerated by temperature changes when temperatures are changed.

Example 1

Next, an example that can be used in the aforesaid embodiment will beexplained. In the following examples, the objective optical element is asingle lens made of polyolefin-based plastic. On the entire surface ofcentral area CN on the optical surface of the objective optical element,there is formed the first optical path difference providing structure.On the entire surface of peripheral area MD on the optical surface,there is formed the second optical path difference providing structure.On the entire surface of most peripheral area OT on the optical surface,there is formed the third optical path difference providing structure.

Further, in Example 1, the first optical path difference providingstructure is of the structure comprising only the first basic structure,which is in a serrated form as shown schematically in FIG. 2( a). Thefirst basic structure representing a diffractive structure in a serratedform is designed to make a diffracted light amount of the first order ofthe diffracted first light flux to be greater than a diffracted lightamount of a diffracted first light flux with any other diffraction ordernumber (including also zero-th order, namely, transmitted light), then,to make a diffracted light amount of the first order of the diffractedsecond light flux to be greater than a diffracted light amount of adiffracted second light flux with any other diffraction order number(including also zero-th order, namely, transmitted light) and to make adiffracted light amount of the first order of the diffracted third lightflux to be greater than a diffracted light amount of a diffracted thirdlight flux with any other diffraction order number (including alsozero-th order, namely, transmitted light).

In Example 1, the second optical path difference providing structure isthe structure wherein the second basic structure and the third basicstructure are overlapped to each other, and it is in a form wherein twotypes of diffractive structures in a serrated form are overlapped toeach other. The second basic structure representing a rougherdiffractive structure in a serrated form is designed to make adiffracted light amount of the fifth order of the first diffracted lightflux to be greater than a diffracted light amount of a diffracted firstlight flux with any other diffraction order number (including alsozero-th order, namely, transmitted light), then, to make a diffractedlight amount of the third order of the diffracted second light flux tobe greater than a diffracted light amount of a diffracted second lightflux with any other diffraction order number (including also zero-thorder, namely, transmitted light) and to make an amount of diffractedlight of the third and second orders of the diffracted third light fluxto be greater than a diffracted light amount of a diffracted third lightflux with any other diffraction order number (including also zero-thorder, namely, transmitted light). The third basic structurerepresenting a finer diffractive structure in a serrated form isdesigned to make a diffracted light amount of the second order of thediffracted first light flux to be greater than a diffracted light amountof the diffracted first light flux with any other order diffractionnumber (including also zero-th order, namely, transmitted light), then,to make a diffracted light amount of the first order of the diffractedsecond light flux to be greater than a diffracted light amount of thediffracted second light flux with any other order diffraction number(including also zero-th order, namely, transmitted light) and to make adiffracted light amount of the first order of the diffracted third lightflux to be greater than a diffracted light amount of the diffractedthird light flux with any other diffraction order number (including alsozero-th order, namely, transmitted light).

In Example 1, the third optical path difference providing structure isof the structure comprising only the second basic structure, and is ofthe form having only the diffractive structure in a serrated form of onetype.

Further, in Example 1, all of the first, second and third optical pathdifference providing structures are provided on the optical surface onthe light source side, and an optical surface of the objective opticalelement on the optical disk side is a refractive surface. Further, inExample 1, the third light flux enters the objective optical element asa slightly finite divergent light.

The lens data of the Example 1 are shown in Table 1. Hereinafter, thepower of 10 will be expressed as by using “E”. For example, 2.5×10⁻³will be expressed as 2.5E−3.

Each of optical surfaces of the objective optical system is formed as anaspheric surface, which has a symmetric shape around the optical axiswith defined by substituting the coefficients shown in the tablesdescribed later into the expression (21).

$\begin{matrix}{{X(h)} = {\frac{\left( {h^{2}/r} \right)}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\left( {h/r} \right)^{2}}}} + {\sum\limits_{i = 0}^{10}{A_{2i}h^{2i}}}}} & (21)\end{matrix}$

Herein, X(h) is the axis along the optical axis (the direction oftraveling light is positive direction), κ is a conical constant, A_(2i)is an aspheric coefficient, h is the height from the optical axis.

Further, the diffractive structure provides the optical path differencewith the light fluxes of respective wavelengths, which is defined bysubstituting the coefficients shown in the tables shown later into theexpression 22. Meanwhile, the optical path difference providingstructures of the present invention can be designed by the way otherthan the following expression (22) and can be expressed by the way otherthan the following expression (22).

$\begin{matrix}{{\Phi (h)} = {\frac{\lambda}{\lambda_{B}} \times {dor} \times {\sum\limits_{i = 0}^{6}{C_{2i}h^{2i}}}}} & (22)\end{matrix}$

Hereupon, λ is the wavelength of the incident light flux, λ_(B) is thedesign wavelength (blaze wavelength), dor is the diffraction order,C_(2i) is the coefficient of the optical path difference function.

Each of FIGS. 6( a), 6(b) and 6(c) shows a vertical spherical aberrationdiagram in Example 1. The numeral 1.0 on the vertical axis in thevertical spherical aberration diagram represents NA 0.85 or the diameterof 3.74 mm. Incidentally, L=0.036 mm and f=2.311 mm hold in Example 1.Therefore, L/f=0.036/2.311=0.016 holds. Further, in Example 1, when awavelength of a light flux for BD is changed by 5 nm, an amount ofchange of third order spherical aberration is −0.134 λrms, an amount ofchange of the fifth order spherical aberration is −0.031 λrms, an amountof change of the seventh order spherical aberration is −0.006 λrms, andan amount of change of the ninth order spherical aberration of is −0.001λrms. Therefore, the total amount of change in the third—ninth ordersspherical aberrations is 0.138 λrms. Further, in Example 1, when awavelength of the light flux for BD is changed by 5 nm, and when amagnification of the fist light flux entering into the objective opticalelement is changed so that the third order spherical aberration may be0, SA5 is −0.009 λrms, SA7 is 0.009 λrms, and SA9 is −0.003 λrms.Therefore, δSAH is 0.013 λrms and δSAH/δλ is 0.0026 (λrms/nm).Incidentally, a using wavelength is 408 nm and an ambient temperature inwavelength characteristics is 25° C.

As for the temperature characteristic of the objective optical elementof Example 1, δSAT1 is +0.0035 WFE λrms/° C. Since the objective opticalelement for the first wavelength provides f′=2.2 mm, δSAT1/f′ is +0.0016WFE λrms/(° C.·mm).

TABLE 1 Focal length of objective lens f₁ = 2.20 mm f₂ = 2.29 mm f₃ =2.31 mm Numerical aperture NA1: 0.85 NA2: 0.66 NA3: 0.51 Magnificationm1: 0 m2: 0 m3: −1/32.8 di ni di ni di ni i^(th) surface ri (408 nm)(408 nm) (658 nm) (658 nm) (785 nm) (785 nm) 0 ∞ ∞ 77.50 1 0.0 0.0 0.0(Aperture (φ3.74 mm) (φ2.94 mm) (φ2.36 mm) diameter) 2 1.39667 2.5101.5596 2.510 1.5406 2.510 1.5372 2-1 1.53466 2-2 1.53516 2-3 1.53572 2-41.53633 2-5 1.53453 2-6 1.53688 2-7 1.53753 2-8 1.53777 2-9 1.54148 2-10 1.50202 3 −3.30470  0.76 0.52 0.23 4 ∞ 0.0875 1.6196 0.600 1.57731.200 1.5709 5 ∞ Surface No. 2 2-1 2-2 2-3 2-4 2-5 Area h ≦ 1.192 1.192≦ h ≦ 1.286 1.286 ≦ h ≦ 1.328 ≦ h ≦ 1.346 1.346 ≦ h ≦ 1.376 1.376 ≦ h ≦1.403 1.328 Aspheric κ −8.10825E−01 −6.18873E−01 −6.23616E−01−6.26957E−01 −6.30541E−01 −6.41040E−01 surface A0 0.00000E+001.59456E−02 1.23373E−02 8.58605E−03 4.90522E−03 7.01584E−04 coefficientA4 1.15964E−02 1.66714E−02 1.66722E−02 1.66725E−02 1.66699E−021.66898E−02 A6 3.38965E−03 6.19009E−03 6.19009E−03 6.19009E−036.19009E−03 6.24994E−03 A8 −1.16842E−03 −2.24233E−03 −2.24233E−03−2.24233E−03 −2.24233E−03 −2.24233E−03 A10 7.48879E−04 1.19717E−031.19717E−03 1.19717E−03 1.19717E−03 1.19717E−03 A12 6.53941E−053.88831E−04 3.88831E−04 3.88831E−04 3.88831E−04 3.88831E−04 A14−1.33307E−04 −3.77956E−04 −3.77956E−04 −3.77956E−04 −3.77956E−04−3.77956E−04 A16 6.35542E−06 8.63256E−05 8.63256E−05 8.63256E−058.63256E−05 8.63256E−05 A18 1.36161E−05 2.28631E−05 2.28631E−052.28631E−05 2.28631E−05 2.28631E−05 A20 −2.44783E−06 −8.07574E−06−8.07574E−06 −8.07574E−06 −8.07574E−06 −8.07574E−06 Optical *1 1/1/12/1/1 2/1/1 2/1/1 2/1/1 2/1/1 path *2 450 nm 395 nm 395 nm 395 nm 395 nm395 nm difference B2 4.81437E−03 −8.14622E−03 −8.14622E−03 −8.14622E−03−8.14622E−03 −8.14622E−03 function B4 −1.59162E−03 3.23575E−033.23575E−03 3.23575E−03 3.23575E−03 3.23575E−03 B6 −4.05599E−046.74397E−04 6.74397E−04 6.74397E−04 6.74397E−04 6.74397E−04 B81.73369E−04 −4.28831E−04 −4.28831E−04 −4.28831E−04 −4.28831E−04−4.28831E−04 B10 −8.10834E−05 1.62584E−04 1.62584E−04 1.62584E−041.62584E−04 1.62584E−04 Surface No. 2-6 2-7 2-8 2-9 2-10 3 Area 1.403 ≦h ≦ 1.426 1.426 ≦ h ≦ 1.446 1.446 ≦ h ≦ 1.464 1.464 ≦ h ≦ 1.481 1.481 ≦h Aspheric κ −6.39313E−01 −6.39425E−01 −6.56555E−01 −6.37857E−01−7.24586E−01 −5.17628E+01 surface A0 −2.67730E−03 −6.60398E−03−1.12205E−02 −1.36156E−02 1.49058E−02 0.00000E+00 coefficient A41.66721E−02 1.66698E−02 1.74520E−02 1.64717E−02 2.08270E−02 9.68152E−02A6 6.21640E−03 6.16708E−03 6.19009E−03 6.19009E−03 3.32455E−03−8.95429E−02 A8 −2.24233E−03 −2.24233E−03 −2.24233E−03 −2.24233E−03−2.60012E−03 6.48819E−02 A10 1.19717E−03 1.19717E−03 1.19717E−031.19717E−03 1.13789E−03 −3.38259E−02 A12 3.88831E−04 3.88831E−043.88831E−04 3.88831E−04 1.17848E−06 9.83086E−03 A14 −3.77956E−04−3.77956E−04 −3.77956E−04 −3.77956E−04 −1.29800E−04 −1.18919E−03 A168.63256E−05 8.63256E−05 8.63256E−05 8.63256E−05 8.13899E−06 0.00000E+00A18 2.28631E−05 2.28631E−05 2.28631E−05 2.28631E−05 1.39700E−050.00000E+00 A20 −8.07574E−06 −8.07574E−06 −8.07574E−06 −8.07574E−06−2.69218E−06 0.00000E+00 Optical *1 2/1/1 2/1/1 2/1/1 2/1/1 5/3/2 path*2 395 nm 395 nm 395 nm 395 nm 408 nm difference B2 −8.14622E−03−8.14622E−03 −8.14622E−03 −8.14622E−03 5.52341E−04 function B43.23575E−03 3.23575E−03 3.23575E−03 3.23575E−03 −1.68198E−04 B66.74397E−04 6.74397E−04 6.74397E−04 6.74397E−04 −5.21270E−05 B8−4.28831E−04 −4.28831E−04 −4.28831E−04 −4.28831E−04 −1.11780E−05 B101.62584E−04 1.62584E−04 1.62584E−04 1.62584E−04 −6.62100E−06 *1:Diffraction order number, *2: Design wavelength

Example 2

Example 2 will be described as follows.

Lens data of Example 2 are shown in Table 2 below. Each of FIGS. 7( a),7(b) and 7(c) shows a vertical spherical aberration diagram in Example2. The numeral 1.0 on the vertical axis in the vertical sphericalaberration diagram represents NA 0.85 or a diameter of 3.74 mm.Incidentally, L=0.098 mm and f=2.334 mm hold in Example 2. Therefore,L/f=0.098/2.334=0.042 holds. Further, in Example 2, when a wavelength ofa light flux for BD is changed by 5 nm, an amount of change of the thirdorder spherical aberration is −0.188 λrms, an amount of change of thefifth order spherical aberration is −0.021 λrms, an amount of change ofthe seventh order spherical aberration of is 0.030 λrms, and an amountof change of the ninth order spherical aberration is −0.016 λrms.Therefore, the total amount of change of the third to ninth orderspherical aberrations of is 0.192 λrms. Further, in Example 2, when awavelength of the light flux for BD is changed by 5 nm, and when amagnification of the first light flux entering to the objective opticalelement is changed so that the third order spherical aberration may be0, SA5 is 0 λrms, SA7 is 0.037 λrms, and SA9 is −0.016 λrms. Therefore,δSAH is 0.042 λrms and δSAH/δλ is 0.0084 (λrms/nm). Incidentally, ausing wavelength is 408 nm and an ambient temperature in wavelengthcharacteristics is 25° C.

As for the temperature characteristic of the objective optical elementof Example 2, δSAT1 is +0.0027 WFE λrms/° C. Since the objective opticalelement for the first wavelength provides f′=2.2 mm, δSAT1/f′ is +0.0012WFE λrms/(° C.·mm).

TABLE 2 Focal length of objective lens f₁ = 2.20 mm f₂ = 2.30 mm f₃ =2.33 mm Numerical aperture NA1: 0.85 NA2: 0.60 NA3: 0.45 Magnificationm1: 0 m2: 0 m3: −1/32.4 di ni di ni di ni i^(th) surface ri (408 nm)(408 nm) (658 nm) (658 nm) (785 nm) (785 nm) 0 ∞ ∞ 77.53 1 0.0 0.0 0.0(Aperture (φ3.74 mm) (φ2.77 mm) (φ2.14 mm) diameter) 2 1.38211 2.4101.5596 2.410 1.5406 2.410 1.5372 2-1 1.51573 2-2 1.55322 2-3 1.51885 3−3.47437  0.82 0.59 0.32 4 ∞ 0.0875 1.6196 0.600 1.5773 1.200 1.5709 5 ∞Surface No. 2 2-1 2-2 2-3 3 Area h ≦ 1.100 1.100 ≦ h ≦ 1.145 1.145 ≦ h ≦1.396 1.396 ≦ h Aspheric κ −6.87288E−01 −8.96269E−01 −6.83136E−01−8.31168E−01 −8.63596E+01 surface coefficient A0 0.00000E+00 4.68657E−025.49909E−02 3.32779E−02 0.00000E+00 A4 6.80300E−03 1.37464E−021.08627E−02 2.16417E−02 9.27005E−02 A6 2.19470E−03 7.35396E−037.34689E−03 3.53299E−03 −9.72902E−02 A8 3.34295E−03 −4.26399E−04−1.03974E−03 2.67295E−03 8.15465E−02 A10 −4.13177E−03 1.22042E−031.22042E−03 −1.77867E−03 −4.46640E−02 A12 1.73984E−03 7.59120E−057.59120E−05 2.04005E−04 1.29730E−02 A14 2.03555E−04 −3.91817E−04−3.91817E−04 2.41464E−04 −1.53003E−03 A16 −2.31775E−04 1.35143E−041.35143E−04 −1.60198E−04 0.00000E+00 A18 −7.16783E−06 9.18829E−069.18829E−06 4.73002E−05 0.00000E+00 A20 8.05067E−06 −6.45098E−06−6.45098E−06 −5.40077E−06 0.00000E+00 Optical *1 1/1/1 2/1/1 2/1/1 5/3/2path *2 530 nm 395 nm 395 nm 408 nm difference B2 9.23389E−03−1.11443E−02 −1.11443E−02 3.00373E−04 function B4 −3.69887E−031.81188E−03 1.81188E−03 −6.55813E−05 B6 1.89068E−03 7.46626E−047.46626E−04 −1.01929E−04 B8 −9.26170E−04 −2.81739E−04 −2.81739E−047.73763E−07 B10 1.14464E−04 9.62982E−05 9.62982E−05 −8.30189E−06 *1:Diffraction order number, *2: Design wavelength

Example 3

Example 3 will be described as follows. Example 3 differs from Example 1in the point that the seventh basic structure is overlapped to a part (aregion closer to the most peripheral area) of the peripheral area and apart (a region closer to the peripheral area) of the most peripheralarea of the objective optical element in order to outreach the thirdlight flux as a flare as far as possible. In other words, there can begiven the second optical path difference providing structure whoseinside area closer to an optical axis has the structure in which thesecond basic structure and the third basic structure are overlapped toeach other, and whose outside area farther than the inside area from theoptical axis has the structure in which the second basic structure, thethird basic structure, and the seventh basic structure are overlapped toeach other. Further there can be given the third optical path differenceproviding structure divided into an inner side area closer to an opticalaxis and an outer side area surrounding the inner side area, in whichthe inner side area is made of the structure wherein the seventh basicstructure and the second basic structure are overlapped to each other,while the outer side area is made of the structure composed only of thesecond basic structure. In Example 3, the seventh basic structure, whichis a binary structure, is designed to make a diffracted light amount ofthe zero-th order of the diffracted first light flux to be greater thana diffracted light amount of a diffracted first light flux with anyother diffraction order number, then, to make a diffracted light amountof the zero-th order of the diffracted second light flux to be greaterthan a diffracted light amount of a diffracted second light flux withany other diffraction order number and to make a diffracted light amountof the ±first order of the diffracted third light flux to be greaterthan a diffracted light amount of a diffracted third light flux with anyother diffraction order number (including also zero-th order, namely,transmitted light).

Lens data of Example 3 are shown in Table 3 below. Each of FIGS. 8( a),8(b) and 8(c) shows a vertical spherical aberration diagram in Example3. The numeral 1.0 on the vertical axis in the vertical sphericalaberration diagram represents NA 0.85 or a diameter of 3.74 mm.Incidentally, L=0.098 mm and f=2.334 mm hold in Example 3. Therefore,L/f=0.098/2.334=0.042 holds. Further, in Example 3, when a wavelength ofa light flux for BD is changed by 5 nm, an amount of change of the thirdorder spherical aberration is −0.188 λrms, an amount of change of thefifth order spherical aberration is −0.021 λrms, an amount of change ofthe seventh order spherical aberration of is 0.030 λrms, and an amountof change of the ninth order spherical aberration is −0.016 λrms.Therefore, the total amount of change of the third—ninth order sphericalaberrations of is 0.192 λrms. Further, in Example 3, when a wavelengthof the light flux for BD is changed by 5 nm, and when a magnification ofthe first light flux entering to the objective optical element ischanged so that the third order spherical aberration may be 0, SA5 is 0λrms, SA7 is 0.037 λrms, and SA9 is −0.016 λrms. Therefore, δSAH is0.042 λrms and δSAH/δλ is 0.0084 (λrms/nm). Incidentally, a usingwavelength is 408 nm and an ambient temperature in wavelengthcharacteristics is 25° C.

As for temperature characteristic of the objective optical element ofthe Example 3, δSAT1 is +0.0027 WFE λrms/° C. Since the objectiveoptical element for the first wavelength provides f′=2.2 mm, δSAT1/f′ is+0.0012 WFE λrms/(° C.·mm).

TABLE 3 Focal length of objective lens f₁ = 2.20 mm f₂ = 2.30 mm f₃ =2.33 mm Numerical aperture NA1: 0.85 NA2: 0.60 NA3: 0.45 Magnificationm1: 0 m2: 0 m3: −1/32.4 di ni di ni di ni i^(th) surface ri (408 nm)(408 nm) (658 nm) (658 nm) (785 nm) (785 nm) 0 ∞ ∞ 77.53 1 0.0 0.0 0.0(Aperture (φ3.74 mm) (φ2.77 mm) (φ2.14 mm) diameter) 2 1.38211 2.4101.5596 2.410 1.5406 2.410 1.5372 2-1 1.51573 2-2 1.55322 2-3 1.51885 2-41.51885 3 −3.47437  0.82 0.59 0.32 4 ∞ 0.0875 1.6196 0.600 1.5773 1.2001.5709 5 ∞ Surface No. 2 2-1 2-2 2-3 2-4 3 Area h ≦ 1.100 1.100 ≦ h ≦1.145 1.145 ≦ h ≦ 1.396 1.396 ≦ h ≦ 1.530 1.530 ≦ h Aspheric κ−6.87288E−01 −8.96269E−01 −6.83136E−01 −8.31168E−01 −8.31168E−01−8.63596E+01 surface A0 0.00000E+00 4.68657E−02 5.49909E−02 3.32779E−023.32779E−02 0.00000E+00 coefficient A4 6.80300E−03 1.37464E−021.08627E−02 2.16417E−02 2.16417E−02 9.27005E−02 A6 2.19470E−037.35396E−03 7.34689E−03 3.53299E−03 3.53299E−03 −9.72902E−02 A83.34295E−03 −4.26399E−04 −1.03974E−03 2.67295E−03 2.67295E−038.15465E−02 A10 −4.13177E−03 1.22042E−03 1.22042E−03 −1.77867E−03−1.77867E−03 −4.46640E−02 A12 1.73984E−03 7.59120E−05 7.59120E−052.04005E−04 2.04005E−04 1.29730E−02 A14 2.03555E−04 −3.91817E−04−3.91817E−04 2.41464E−04 2.41464E−04 −1.53003E−03 A16 −2.31775E−041.35143E−04 1.35143E−04 −1.60198E−04 −1.60198E−04 0.00000E+00 A18−7.16783E−06 9.18829E−06 9.18829E−06 4.73002E−05 4.73002E−05 0.00000E+00A20 8.05067E−06 −6.45098E−06 −6.45098E−06 −5.40077E−06 −5.40077E−060.00000E+00 Optical *1 1/1/1 2/1/1 2/1/1 5/3/2 5/3/2 path *2 530 nm 395nm 395 nm 408 nm 408 nm difference B2 9.23389E−03 −1.11443E−02−1.11443E−02 3.00373E−04 3.00373E−04 function 1 B4 −3.69887E−031.81188E−03 1.81188E−03 −6.55813E−05 −6.55813E−05 B6 1.89068E−037.46626E−04 7.46626E−04 −1.01929E−04 −1.01929E−04 B8 −9.26170E−04−2.81739E−04 −2.81739E−04 7.73763E−07 7.73763E−07 B10 1.14464E−049.62982E−05 9.62982E−05 −8.30189E−06 −8.30189E−06 Optical *1 0/0/1 or −10/0/1 or −1 path *2 785 nm 785 nm difference B2 1.60000E−02 1.60000E−02function 2 B4 1.00000E−03 1.00000E−03 B6 5.00000E−04 5.00000E−04 B80.00000E+00 0.00000E+00 B10 0.00000E+00 0.00000E+00 *1: Diffractionorder number, *2: Design wavelength

Other various embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

1. An optical pickup apparatus for recording and/or reproducing information for an optical disk, the optical pickup apparatus comprising: a first light source for emitting a first light flux having a first wavelength λ1; a second light source for emitting a second light flux having a second wavelength λ2 (λ2>λ1); a third light source for emitting a third light flux having a third wavelength λ3 (λ3>λ2); and an objective optical element for converging the first light flux onto an information recording surface of a first optical disk having a protective substrate with a thickness t1, for converging the second light flux onto an information recording surface of a second optical disk having a protective substrate with a thickness t2 (t1≦t2), and for converging the third light flux onto an information recording surface of a third optical disk having a protective substrate with a thickness t3 (t2<t3), wherein the optical pickup apparatus records and/or reproduces information by converging the first light flux onto the information recording surface of the first optical disk, by converging the second light flux onto the information recording surface of the second optical disk, and by converging the third light flux onto the information recording surface of the third optical disk, wherein the objective optical element comprises an optical surface comprising a central area and a peripheral area surrounding the central area, the central area comprises a first optical path difference providing structure, and the peripheral area comprises a second optical path difference providing structure, wherein the objective optical element converges the first light flux which passes through the central area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the first optical disk, the objective optical element converges the second light flux which passes through the central area of the objective optical element onto the information recording surface of the second optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the second optical disk, the objective optical element converges the third light flux which passes through the central area of the objective optical element onto the information recording surface of the third optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the third optical disk, the objective optical element converges the first light flux which passes through the peripheral area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the first optical disk, and the objective optical element converges the second light flux which passes through the peripheral area of the objective optical element onto the information recording surface of the second optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the second optical disk, wherein the third light flux which passes through the first optical path difference providing structure forms a first best focus in which the third light flux forms a spot having a smallest diameter, the third light flux which passes through the first optical path difference providing structure forms a second best focus in which the third light flux forms a spot having a second smallest diameter, and the first best focus and the second best focus satisfy a following expression: 0<L/f<0.05, where f (mm) is a focal length of the objective optical element for the third light flux which passes through the first optical path difference structure and forms the first best focus, and L (mm) is a distance between the first best focus and the second best focus, wherein the third light flux which passes through the objective optical element forms a spot on the information recording surface of the third optical disk, and the spot comprises, in order from a center to an outside of the spot when viewing the spot from a direction of an optical axis of the objective optical element: a central spot portion having a highest light density; an intermediate spot portion having a lower light density than the central spot portion; and a peripheral spot portion having a higher light density than the intermediate spot portion and having a lower light density than the central spot portion, wherein the central spot portion is used for recording and/or reproducing information for the third optical disk, and the intermediate spot portion and the peripheral spot portion are not used for recording and/or reproducing information for the third optical disk, and wherein the peripheral spot portion is formed on the information recording surface of the third optical disk by the third light flux which passes through the second optical path difference providing structure of the objective optical element.
 2. The optical pickup apparatus of claim 1, satisfying following expressions: δSAH/δλ≦0.010 (λrms/nm), and δSAH=√((δSA5)²+(δSA7)²+(δSA9)²), where δSA5 is a fifth order spherical aberration generated when information is recorded and/or reproduced for the first optical disk using a light flux with a wavelength λx which is shifted from a using wavelength of 408 nm at a magnification making a third order spherical aberration SA3 zero, the magnification being the magnification of the light flux with the wavelength of λx for the objective optical element, δSA7 is a seventh order spherical aberration generated when information is recorded and/or reproduced for the first optical disk using a light flux with the wavelength λx at a magnification making a third order spherical aberration SA3 zero, the magnification being the magnification of the light flux with the wavelength of λx for the objective optical element, δSA9 is a ninth order spherical aberration generated when information is recorded and/or reproduced for the first optical disk using a light flux with the wavelength λx at a magnification making a third order spherical aberration SA3 zero, the magnification being the magnification of the light flux with the wavelength of λx for the objective optical element, and δλ is an absolute value of a difference between 408 nm and λx nm.
 3. The optical pickup apparatus of claim 1, wherein according to a graph whose horizontal axis represents a distance from an optical axis in a direction perpendicular to the optical axis of the objective optical element, and whose vertical axis represents an optical path difference of the first light flux provided by the objective optical element when the first light flux passes through the objective optical element, and the graph comprises a discontinuity at a wavelength which is shifted by 5 nm from a designed wavelength of the first wavelength of the objective optical element, and a gap width of the optical path difference at the discontinuity is 0 or more, and 0.2 λ1 or less.
 4. The optical pickup apparatus of claim 1, satisfying η13≧40%, where η13 is an efficiency of a light utilization of the third light flux on the central area.
 5. The optical pickup apparatus of claim 1, wherein the spot formed by the third light flux in the first best focus is used for recording and/or reproducing information for the third optical disk, and the spot formed by the third light flux in the second best focus is not used for recording and/or reproducing information for the third optical disk.
 6. The optical pickup apparatus of claim 1, wherein the optical surface of the objective optical element further comprises a most peripheral area surrounding the peripheral area and being a refractive surface.
 7. The optical pickup apparatus of claim 1, wherein the optical surface of the objective optical element further comprises a most peripheral area surrounding the peripheral area and comprising a third optical path difference providing structure.
 8. The optical pickup apparatus of claim 6, wherein the objective optical element converges the first light flux passing through the most peripheral area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information for the first optical disk.
 9. The optical pickup apparatus of claim 7, wherein the objective optical element converges the first light flux passing through the most peripheral area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information for the first optical disk.
 10. The optical pickup apparatus of claim 1, wherein the first optical path difference providing structure corrects a spherical aberration for the first light flux and the second light flux each passing through the first optical path difference providing structure, the spherical aberration being caused by a difference between the thickness t1 of the protective substrate of the first optical disk and the thickness t2 of the protective substrate of the second optical disk.
 11. The optical pickup apparatus of claim 1, wherein the first optical path difference providing structure corrects a spherical aberration for the first light flux and the third light flux each passing through the first optical path difference providing structure, the spherical aberration being caused by a difference between the thickness t1 of the protective substrate of the first optical disk and the thickness t3 of the protective substrate of the third optical disk.
 12. The optical pickup apparatus of claim 1, wherein the second optical path difference providing structure corrects a spherical aberration for the first light flux and the second light flux each passing through the second optical path difference providing structure, the spherical aberration being caused by a difference between the thickness t1 of the protective substrate of the first optical disk and the thickness t2 of the protective substrate of the second optical disk.
 13. The optical pickup apparatus of claim 1, wherein the second optical path difference providing structure corrects a spherochromatism or a spherical aberration for the first light flux and the second light flux each passing through the second optical path difference providing structure, the spherochromatism being caused by a slight fluctuation of the wavelength of the first light flux or the second light flux, the spherical aberration being caused by a temperature change in the objective optical element.
 14. The optical pickup apparatus of claim 1, satisfying following expressions: −0.02<m1<0.02, and −0.02<m2<0.02, where m1 is a magnification of the first light flux entering into the objective optical element, and m2 is a magnification of the second light flux entering into the objective optical element.
 15. The optical pickup apparatus of claim 14, further satisfying a following expression: −0.02<m3<0.02, where m3 is a magnification of the third light flux entering into the objective optical element.
 16. The optical pickup apparatus of claim 14, further satisfying a following expression: −0.10<m3<0.00 where m3 is a magnification of the third light flux entering into the objective optical element.
 17. The optical pickup apparatus of claim 1, wherein the objective optical element is a single lens.
 18. The optical pickup apparatus of claim 1, further comprising an opening limiting element arranged between the objective optical element and the first light source, between the objective optical element and the second light source, and between the objective optical element and the third light source in a common optical path of the first, second, and third light fluxes, wherein the opening limiting element comprises a first area closest to an optical axis, and a second area arranged farther position from the optical axis than the first area, the first area transmits the first light flux, the second light flux, and the third light flux, the second area transmits the first light flux and the second light flux and does not converge the third light flux at a light converging position of the third light flux which passes through the first area of the opening limiting element and the objective optical element, and the third light flux which passes through the first area enters into the central area of the objective optical element.
 19. The optical pickup apparatus of claim 18, further comprising a λ/4 wavelength plate, wherein the opening limiting element is integrally formed in one body with the λ/4 wavelength plate.
 20. The optical pickup apparatus of claim 18, satisfying 0.02≦L/f<0.05.
 21. An objective optical element for use in an optical pickup apparatus, the optical pickup apparatus comprising: a first light source for emitting a first light flux having a first wavelength λ1; a second light source for emitting a second light flux having a second wavelength λ2 (λ2>λ1); and a third light source for emitting a third light flux having a third wavelength λ3 (λ3>λ2), and conducting recording and/or reproducing information using the first light flux for a first optical disk having a protective substrate with a thickness t1, conducting recording and/or reproducing information using the second light flux for a second optical disk having a protective substrate with a thickness t2 (t1≦t2), and conducting recording and/or reproducing information using the third light flux for a third optical disk having a protective substrate with a thickness t3 (t2<t3), the objective optical element comprising: an optical surface comprising a central area and a peripheral area surrounding the central area, wherein the central area comprises a first optical path difference providing structure, the peripheral area comprises a second optical path difference providing structure, wherein the objective optical element converges the first light flux which passes through the central area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the first optical disk, the objective optical element converges the second light flux which passes through the central area of the objective optical element onto the information recording surface of the second optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the second optical disk, the objective optical element converges the third light flux which passes through the central area of the objective optical element onto the information recording surface of the third optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the third optical disk, the objective optical element converges the first light flux which passes through the peripheral area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the first optical disk, and the objective optical element converges the second light flux which passes through the peripheral area of the objective optical element onto the information recording surface of the second optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the second optical disk, wherein the third light flux which has passed through the first optical path difference providing structure forms a first best focus in which the third light flux forms a spot having a smallest diameter, the third light flux which passes through the first optical path difference providing structure forms a second best focus in which the third light flux forms a spot having a second smallest diameter, and the first best focus and the second best focus satisfy a following expression: 0<L/f<0.05, where f (mm) is a focal length of the objective optical element for the third light flux which passes through the first optical path difference structure and forms the first best focus, and L (mm) is a distance between the first best focus and the second best focus, wherein the third light flux which passes through the objective optical element forms a spot on the information recording surface of the third optical disk, and the spot comprises, in order from a center to an outside of the spot when viewing the spot from a direction of an optical axis of the objective optical element: a central spot portion having a highest light density; an intermediate spot portion having a lower light density than the central spot portion; and a peripheral spot portion having a higher light density than the intermediate spot portion and having a lower light density than the central spot portion, wherein the central spot portion is used for recording and/or reproducing information for the third optical disk, and the intermediate spot portion and the peripheral spot portion are not used for recording and/or reproducing information for the third optical disk, and wherein the peripheral spot portion is formed on the information recording surface of the third optical disk by the third light flux which has passed through the second optical path difference providing structure of the objective optical element.
 22. The objective optical element of claim 21, satisfying following expressions: δSAH/δλ≦0.010 (λrms/nm), and δSAH=√((δSA5)²+(δSA7)²+(δSA9)²), where δSA5 is a fifth order spherical aberration generated when information is recorded and/or reproduced for the first optical disk using a light flux with a wavelength λx which is shifted from a using wavelength of 408 nm at a magnification making a third order spherical aberration SA3 zero, the magnification being the magnification of the light flux with the wavelength of λx for the objective optical element, δSA7 is a seventh order spherical aberration generated when information is recorded and/or reproduced for the first optical disk using a light flux with the wavelength λx at a magnification making a third order spherical aberration SA3 zero, the magnification being the magnification of the light flux with the wavelength of λx for the objective optical element, δSA9 is a ninth order spherical aberration generated when information is recorded and/or reproduced for the first optical disk using a light flux with the wavelength λx at a magnification making a third order spherical aberration SA3 zero, the magnification being the magnification of the light flux with the wavelength of λx for the objective optical element, and δλ is an absolute value of a difference between 408 nm and λx nm.
 23. The objective optical element of claim 21, wherein the optical pickup apparatus provides a graph whose horizontal axis represents a distance from an optical axis in a direction of a radius of the objective optical element, and whose vertical axis represents an optical path difference of the first light flux provided by the objective optical element when the first light flux passes through the objective optical element, and the graph comprises a discontinuity at a wavelength which is shifted by 5 nm from a designed wavelength of the first wavelength of the objective optical element, and a gap width of the optical path difference at the discontinuity is 0 or more, and 0.2 λ1 or less.
 24. The objective optical element of claim 21, satisfying η13≧40%, where η13 is an efficiency of a light utilization of the third light flux on the central area.
 25. The objective optical element of claim 21, wherein the spot formed by the third light flux in the first best focus is used for recording and/or reproducing information for the third optical disk, and the spot formed by the third light flux in the second best focus is not used for recording and/or reproducing information for the third optical disk.
 26. The objective optical element of claim 21, wherein the optical surface of the objective optical element further comprises a most peripheral area surrounding the peripheral area and being a refractive surface.
 27. The objective optical element of claim 21, wherein the optical surface of the objective optical element further comprises a most peripheral area surrounding the peripheral area and comprising a third optical path difference providing structure.
 28. The objective optical element of claim 26, wherein the objective optical element converges the first light flux passing through the most peripheral area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information for the first optical disk.
 29. The objective optical element of claim 27, wherein the objective optical element converges the first light flux passing through the most peripheral area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information for the first optical disk.
 30. The objective optical element of claim 21, wherein the first optical path difference providing structure corrects a spherical aberration for the first light flux and the second light flux each passing through the first optical path difference providing structure, the spherical aberration being caused by a difference between the thickness t1 of the protective substrate of the first optical disk and the thickness t2 of the protective substrate of the second optical disk.
 31. The objective optical element of claim 21, wherein the first optical path difference providing structure corrects a spherical aberration for the first light flux and the third light flux each passing through the first optical path difference providing structure, the spherical aberration being caused by a difference between the thickness t1 of the protective substrate of the first optical disk and the thickness t3 of the protective substrate of the third optical disk.
 32. The objective optical element of claim 21, wherein the second optical path difference providing structure corrects a spherical aberration for the first light flux and the second light flux each passing through the second optical path difference providing structure, the spherical aberration being caused by a difference between the thickness t1 of the protective substrate of the first optical disk and the thickness t2 of the protective substrate of the second optical disk.
 33. The objective optical element of claim 21, wherein the second optical path difference providing structure corrects a spherochromatism for the first light flux and the second light flux each passing through the second optical path difference providing structure, the spherochromatism being caused by a slight fluctuation of the wavelength of the first light flux or the second light flux, the spherical aberration being caused by a temperature change in the objective optical element.
 34. The objective optical element of claim 21, wherein the objective optical element is a single lens.
 35. An optical information recording and/or reproducing apparatus, comprising: an optical pickup apparatus comprising: a first light source for emitting a first light flux having a first wavelength λ1; a second light source for emitting a second light flux having a second wavelength λ2 (λ2>λ1); a third light source for emitting a third light flux having a third wavelength λ3 (λ3>λ2); and an objective optical element converging the first light flux onto an information recording surface of a first optical disk having a protective substrate with a thickness t1, for converging the second light flux onto an information recording surface of a second optical disk having a protective substrate with a thickness t2 (t1≦t2), and for converging the third light flux onto an information recording surface of a third optical disk having a protective substrate with a thickness t3 (t2<t3), wherein the optical pickup apparatus records and/or reproduces information by converging the first light flux onto the information recording surface of the first optical disk, by converging the second light flux onto the information recording surface of the second optical disk, and by converging the third light flux onto the information recording surface of the third optical disk, wherein the objective optical element comprises an optical surface comprising a central area and a peripheral area surrounding the central area, the central area comprises a first optical path difference providing structure, and the peripheral area comprises a second optical path difference providing structure, wherein the objective optical element converges the first light flux which passes through the central area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the first optical disk, the objective optical element converges the second light flux which passes through the central area of the objective optical element onto the information recording surface of the second optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the second optical disk, the objective optical element converges the third light flux which passes through the central area of the objective optical element onto the information recording surface of the third optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the third optical disk, the objective optical element converges the first light flux which passes through the peripheral area of the objective optical element onto the information recording surface of the first optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the first optical disk, and the objective optical element converges the second light flux which passes through the peripheral area of the objective optical element onto the information recording surface of the second optical disk so that the optical pickup apparatus can record and/or reproduce information on the information recording surface of the second optical disk, wherein the third light flux which passes through the first optical path difference providing structure forms a first best focus in which the third light flux forms a spot having a smallest diameter, the third light flux which passes through the first optical path difference providing structure forms a second best focus in which the third light flux forms a spot having a second smallest diameter, and the first best focus and the second best focus satisfy a following expression: 0<L/f<0.05, where f (mm) is a focal length of the objective optical element for the third light flux which passes through the first optical path difference structure and forms the first best focus, and L (mm) is a distance between the first best focus and the second best focus, wherein the third light flux which passes through the objective optical element forms a spot on the information recording surface of the third optical disk, and the spot comprises, in order from a center to an outside of the spot when viewing the spot from a direction of an optical axis of the objective optical element: a central spot portion having a highest light density; an intermediate spot portion having a lower light density than the central spot portion; and a peripheral spot portion having a higher light density than the intermediate spot portion and having a lower light density than the central spot portion, and wherein the central spot portion is used for recording and/or reproducing information for the third optical disk, and the intermediate spot portion and the peripheral spot portion are not used for recording and/or reproducing information for the third optical disk, and wherein the peripheral spot portion is formed on the information recording surface of the third optical disk by the third light flux which passes through the second optical path difference providing structure of the objective optical element. 